Living in the Universe

Methane on a Distant World

Fourth Week of March 2008

Last week saw the exciting detection of the molecule methane for the first time in the atmosphere of a planet outside the solar system. The finding comes from the extrasolar system called HD 189733. It’s a system that’s been in the news before because the star has a gaseous hot Jupiter locked in a tight orbit around it.  They are both 63 light years away. According to the team, who are from NASA’s Jet Propulsion Lab and the University College of London, the observations decisively show that methane is present, in addition to water. The same team reported last year that they’d identified water vapor in the atmosphere of this planet using a similar technique.

This story conjures up thoughts of cow farts on Alpha Centauri, but although we earthlings associate methane with gassy cows and ruminants, it is a common and perfectly non-biological constituent of other atmospheres in the solar system, including Mars and Titan, as well as the gas giants Jupiter, Saturn, Uranus, and Neptune. There’s been a debate in the past few years over the presence of methane in the thin Martian atmosphere and whether that methane could point to microbial life under the surface. But the methane on Mars is at such a low concentration—a few parts per billion—that it doesn’t decisively indicate biological metabolism at work. It could easily come from geological processes. Researchers believe that methane and water will be common constituents of planetary atmospheres outside the solar system; nonetheless, it’s an important ingredient.

Methane as a tracer is part of a larger debate over biomarkers. Which atmospheric tracers are most likely to indicate biology? Measuring the relative abundance of elements in an atmosphere allows researchers to infer details about how the planet has formed and its weather patterns. The planet and star HD 189733 present an exciting opportunity because it’s one of the few extrasolar planets suited to such measurements.  It’s a transiting exoplanet, which means it crosses in front of its parent star, and because it’s on such a tight orbit it does so every 2.2 days. And because it’s a big planet, Jupiter-sized, it blocks two percent of the parent star’s light each time it does so.

The eclipse technique relies on the fact that the planetary atmosphere is backlit by the star. Every molecule absorbs light most strongly at particular wavelengths so by measuring the amount of light blocked at different wavelengths in the planetary atmosphere, researchers can infer its composition.  There’s extra absorption at the particular wavelengths corresponding to methane when the planet is in transit.  Researchers reported their results in the journal Nature. They saw the telltale patterns of both methane and water. The instrument used was NICMOS, the Near Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope, which was the same instrument they’d used to detect water the years before.

We might wonder if this technique can find interesting gases in planets more like Earth, but the technique is difficult. Jupiter, remember, is ten times larger than the Earth so if an Earth-like planet passes in front of its star it blocks ten squared, or a hundred times, less light.  In other words the technique would have to be a hundred times more sensitive to find methane in an Earth-like planet, assuming we can find Earth-like planets.

In an accompanying commentary by planetary scientist Adam Showman, who works across the street at the Lunar and Planetary Lab, the implication is that a third constituent carbon monoxide is waiting to be found in the atmosphere of this planet. Planets are presumed to form from the same material as stars but Showman notes that the intensity of the methane absorption implies that the planet has a low methane-to-hydrogen ratio, no more than five parts per hundred thousand, which is only ten percent of its parent star. The scorching temperature of this planet, around a thousand Kelvin or thirteen hundred degrees Fahrenheit, may cause the carbon in its atmosphere to prefer joining oxygen as carbon monoxide instead of forming methane. “Finding the carbon monoxide and mapping its distribution with that of methane will illuminate the planet’s exotic weather patterns,” according to Showman. He says, “These are exciting times for studies of extrasolar planets.  Researchers are finally moving beyond simply discovering them to truly characterizing them as worlds.”

This is slow and painstaking research, but over the next decade we can anticipate that dozens of extrasolar planets will have spectral diagnostics and we’ll begin to truly understand the nature of their atmospheres. And that is an important step along the road to using biomarkers to detect microbial life on those planets.

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There’s No Place Like Home, Yet

Fourth Week of February 2008

There’s no place like home, yet, but astronomers are getting that much closer. Somewhere over the rainbow astronomers anticipate finding analogs of the solar system and clones of the planet Earth. Since 1995, over two hundred and fifty planets have been found beyond the solar system—exoplanets as they’re called—but very few of them are in systems that even faintly resemble our own. In many cases giant Jupiter-like planets whirl around in orbits that are closer to their star than Mercury is to the Sun. But are these really typical of what’s out there in the universe? Also, almost all of these planets were discovered by a Doppler wobble method in which astronomers measure the gravitational tug of planets on their parent star as they whirl around it. This technique is particularly sensitive to massive planets close to their stars.

So there was great excitement last week when astronomers said they had found a miniature version of our own solar system five thousand light years away across the galaxy. It’s the first planetary system that really looks like our own, with outer giant planets and room for small inner planets, although no terrestrial planets have been found yet. Scott Gaudi, assistant professor at Ohio State, led the international team of 69 professional and amateur astronomers who announced the discovery in a big news conference. He said, “It looks like a scale model of our solar system.” Their results are being published in the journal Science.

They say it means that our solar system may be more typical of planetary systems across the universe than have been thought. In this new system a planet about two-thirds the mass of Jupiter and another about ninety percent of the mass of Saturn are orbiting a reddish star about half the distances that Jupiter and Saturn circle our own Sun. The star itself is about half the mass of the Sun, a little cooler and a little redder. Neither of these two giant planets is a likely abode for life as we know it, but Dr. Gaudi and the team say that warm, rocky planets suitable for life could easily exist undetected in the inner parts of the system.

One of the exciting things about this discovery is the way it was made. It used a different technique than the Doppler technique, one that favors planets more distant from the star. It’s based on a trick of gravity called microlensing. If as stars and planets move to-and-fro in space two of them should become almost perfectly aligned with the Earth, the gravity of the nearby star can bend and magnify the light from the more distant one, causing it to get brighter for a few weeks or months. If the alignment is perfect, any big planets attending the nearby star will also be part of this process, adding their own little boost to the distant starlight. You never actually see the distant planet or the distant star. This is an indirect technique for finding planets and it’s a one-off; the signal never repeats.

That’s exactly what happened on March 28 a year or so ago (with the research paper just being published last week), when a star 5000 light years away in the constellation Scorpius began to pass in front of one twenty-one thousand light years more distant, causing it to brighten and then fade. The event was picked up by OGLE, the Optical Gravitational Lensing Experiment, which is a worldwide collaboration of observers who watch out for such rare events. OGLE immediately issued a worldwide call for anyone who could point a telescope at what is one of their best candidate stars around which to find a planet.

For the next two weeks the pace was frenetic. Among those who provided crucial data, and who appear as lead authors on the paper in Science, are a pair of amateur astronomers from New Zealand, Jennie McCormick and Grant Christie, both of whom are members of another group called the Microlensing Follow-Up Network, or MicroFUN. Jennie McCormick had already been involved in the discovery of a planet several years ago by the microlensing technique. She’s both an amateur astronomer and a full-time mom, and she was quoted as saying, “It just goes to show you can be a mom, you can work full-time, and you can still go out there and find planets.” Exactly.

To the experimenter’s surprise, by clever manipulation of their data, they were able to dig out not just the masses of the star and its 2 planets but rough approximations of their orbits, which confirmed its similarity to our solar system. Microlensing is poised to become a new, powerful tool in the planet hunter’s arsenal. These two planets are only the fifth and sixth to be discovered by microlensing so far, and the Scorpius event is the first where the alignment of the stars was close enough for astronomers to detect more than one planet at once. But this success indicates that microlensing is a technique for the future, and it may just become the way we find places like home.

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A Familiar Solar System

First Week of November 2007

There’s no place like home, but we’re finally beginning to find places beyond the Solar System that resemble home. Astronomers reported this week that there are at least five planets circling a star known as 55 Cancri in the constellation of Cancer, whereas only four had been known before, making this system the most extensive set of planets yet found outside our own.

This is very exciting. Over two hundred and fifty extrasolar planets have been discovered, but most of them are single planets, Jupiter-sized or larger. The set of planets around 55 Cancri really resembles something like our solar system where one planet that we’re quite partial to is in the so-called habitable zone, just warm enough for liquid water but not so hot that it evaporates. The planet that’s in the habitable zone in 55 Cancri, however, is forty-five times the mass of the Earth, so this planet is more like Neptune or Saturn than the Earth and would probably be a deadly environment for any kind of life that we know.

Debra Fischer from San Francisco State University led the team that discovered this fifth planet around this one system. She says, “It’s a system that appears to be packed with planets.” Another of the team members, Geoff Marcy, isn’t jaded by his part in discovering dozens of extrasolar planets; he said he was jumping out of his socks. “We now know the Sun and its family of planets is not unusual,” said Geoff. The discovery of this signal and this extrasolar planet is a good sign that astronomers will continue to find new planets in the systems already discovered, adding numbers and adding lower mass objects much closer to the Earth in size. Jonathan Lunine, who is a colleague of mine at the University of Arizona, said that astronomers were on the verge of answering a question posed by Albertus Magnus, a medieval German philosopher and priest, who wondered whether there was but one world or many worlds. “As we now know,” Dr. Lunine said, “the universe is lonely; how far we live from the distant stars.”

In this past decade, having found all these extrasolar planets, the technology and techniques have improved, and planet hunters have been moving down the scale from Jupiter-mass planets to some that are just a few times the mass of the Earth. Detecting Earth clones is beyond the limits of the Doppler method and will await future space-based missions. 55 Cancri was one of the first exoplanets discovered in 1996. Debra Fischer and her colleagues have been observing the system now for eighteen years, adding more planets to the list as they make their presence known. The outermost and heaviest planet in that system is four times as massive as Jupiter and circles at a distance of five hundred million miles which is only slightly farther than Jupiter in our own system. It takes fourteen years to complete an orbit. The star’s three innermost planets all circle more tightly than our Mercury at distances from twenty to three and a half million miles. The closest of the three is also the smallest, only eighteen times as massive as the Earth; it’s surely a scorched and nasty place to live.

The new planet, which Dr. Fischer calls one of the more annoying planets because it resisted being folded into their mathematical models for such a long time, basks in the lukewarm light of its star at a distance of about seventy million miles, and it takes two hundred and sixty days to complete one orbit. Although the planet itself is likely to be too massive for life, Dr. Marcy thinks that the planet could harbor rocky moons just as Jupiter and Saturn and Neptune in our solar system do, and these moons will be warmed to the same lukewarm temperature as the Earth. The moons would have to be as massive as Mars, however, in order to keep their water from escaping into empty space.

The discovery of this new planet highlights the difficulty and the sophistication of the techniques that are now used to find yet more planets in existing systems. You can think of these lower mass planets in a system that already has a massive planet as overtones, like musical notes that are found on the Doppler curve. Essentially, astronomers use a harmonic analysis that’s very musical in nature to find smaller mass planets in systems already known to have large mass planets.

We like to think that some of these planets that are being discovered could host life, but of course we don’t really know. It would take spectroscopy of the feeble reflected light from the planets to look for trace gases that might indicate biology; these indicators are called biomarkers and such observations are very difficult. The discovery of true Earth clones is probably five to ten years away, but a system with five planets is so strikingly similar to our own solar system that it tells us that the discovery and study of extrasolar planets is moving into a new and exciting phase.

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Lonely Stars and Planets

Fourth Week of September 2007

We imagine the skies that we see would be similar to the skies that other intelligent creatures around the galaxy might see, but that may not be the case. There may be very lonely skies and very lonely creatures out there in the galaxy. This past week, astronomers have found evidence that stars have formed in a long tail of gas that extends well outside its parent galaxy. The discovery suggests that orphan stars may be much more prevalent than previously thought.

The comet-like tail was discovered in the X-ray light with the NASA Chandra Observatory and in the optical light with the Southern Astrophysical Research Telescope in Chile. The feature extends for more than two hundred thousand light years and was created as gas was stripped from a galaxy called ESO137001 that is plunging towards the center of a giant cluster of galaxies. This is one of the longest tails that anyone has ever seen, and it’s a wake of creation, not destruction, because observations indicate that the gas in the tail has formed millions of stars. Because the large amounts of gas and dust needed to form stars are typically only found within galaxies, astronomers previously thought it was unlikely that stars could form outside a galaxy.

Team member Megan Donahue says, “This isn’t the first time stars have been seen to form between galaxies, but the number of stars forming here is unprecedented.” The evidence includes twenty-nine regions of ionized hydrogen glowing in optical light thought to be found nearby new stars. These regions are all downstream of the galaxy located in or near the tail. Two Chandra X-ray sources are seen near these regions, another indication of star formation activity. The researchers believe the orphan stars formed within the last ten million years which makes them very young stars. Mark Voit, another member of the team, says, “By our galactic standards, these stars are extremely lonely. If life were to form out there on a planet a few billion years from now, they would have incredibly dark skies.”

The gas that formed the stars was stripped out of its parent galaxy by pressure induced by the motion of the galaxy through the multi-million degree gas that pervades the intergalactic space. Eventually most of the gas will be scoured from the galaxy, depleting raw material for new stars and stopping star formation in the galaxy. This process may represent an important but short-lived stage in the transformation of a galaxy. Although apparently rare in the present-day universe, tails of gas and orphan stars may have been much more common billions of years ago when galaxies were younger.

This discovery leads to a speculation: what type of life might form, and what skies and vision of their galaxy might life have in other neighborhoods. Our Sun is an undistinguished star living in an unremarkable suburb of the Milky Way, but what if we were transported to a different part of the Milky Way and a different star? If we examine our expectations for life beyond Earth, we’re bounded not only by the history of our planet but also by our particular cosmic environment. So let’s do the visualization.

Our first stop isn’t far from home as the crow flies, eighteen hundred light years. We’re in Orion, a bustling region of star formation that traces a spiral arm of the Milky Way. Our journey through a wormhole dumps us near a hot, young star a hundred times brighter than the Sun. Cobwebs of gas drape the sky, and most stars look slightly red due to the gauze of dust. Four trapezium stars blaze brightly, and other massive stars litter the sky. There’s evidence of past supernovae and heavy elements have been ejected liberally. With so much material for planets, most stars have a dozen or more. On the other hand, many stars live less than a hundred million years, and those that live longer must contend with the violent death of their massive neighbors. Among the habitable planets there are many truncated biological experiments. The cosmic environment favors life with a fast evolutionary clock and life that develops below water and lives in rock.

Our next hop through the wormhole drops us near the urban center of the galaxy where the density of stars is a thousand times higher than near the Sun. The entire scene is lit brighter than a full moon sky on Earth, an unfamiliar sight. Where stars are crowded the tightest, the sky crackles with the high-energy radiation from the heart of darkness in the Milky Way galaxy, a supermassive black hole. A cool dwarf hangs like a blood orange over our heads. It has six planets on tight orbits, two of which are in the habitable zone. This seems like a promising environment for life. There’s a lot of iron and silicon for building planets and lots of carbon for life. Planets are drifting among the stars, ripped from their gravity moorings from stellar encounters. A few are shrouded in thick atmospheres and massive enough not to need external life support from a star. With a high density and many planets per star, the spread of life is guaranteed. Life-bearing rocks are routinely ejected from planet surfaces by impacts resulting in an inefficient but extensive shuttle system for life. The high degree of stellar concentration means the signaling time between intelligent civilizations, if they exist, is short. If there is an interstellar Internet anywhere in the galaxy, this is the place.

Our last jump takes us thirty thousand light years into the halo, and this is similar to the environment we started the story with. There’s dark matter here of course, like there is everywhere, but no gas or dust and very few stars. The two-armed spiral of our disk is laid out below like a Persian rug, blue light knots of star formation set into a sparkling yellow star field. The white dwarf nearby is hot, titanium white. Formed a long time ago with not much grist for planets, it has three the size of Mercury, and they’re all long dead. The nearest star is a hundred light years away. Biology is sprinkled lightly in the attic of the galaxy. It’s lonely here and a shame that such a gorgeous view is wasted.

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Giant Outer Exoplanets are Rare

Third Week of July 2007

I’m pleased to be able to report this week on major research done by my colleagues at the University of Arizona, in particular by professor Laird Close in the department, and by one of our best young graduate students, Beth Biller. These astronomers and others in their team have used powerful new telescopes in Arizona and Chile to survey planets around other stars, and they’ve discovered that extrasolar planets more massive than Jupiter are extremely rare in other outer solar systems.

Unlike most of the studies done for extrasolar planets that have led to their discovery using the Doppler method, this was an imaging survey using highly sophisticated techniques to cheat the atmosphere and its blurring and produce exceptionally sharp images that would allow the detection of a planet near a distant star. These astronomers just concluded a benchmark three-year survey using direct detection techniques better than anyone has used before to look at fifty-four young nearby stars. These stare are among the best candidates for having detectable Jupiter-like planets at distances beyond five astronomical units or the distance between Jupiter and the Sun. As a reminder, One A.U. is the distance between the Earth and the Sun.

So far we’ve found over two hundred and thirty planets around other stars. Many of them are super-Jupiters orbiting very close to their parent stars. Scientists have written over two thousand papers about these giant Jupiter-like planets within a few Earth-to-Sun distances of their stars. However, the radial velocity method presently used is most sensitive to planets close to their stars moving on fairly rapid orbits. The technique has not been going long enough to reveal extrasolar planets further out. Remember that Jupiter in its orbit of the Sun takes twelve years. So even more distant planets would take decades to complete an orbit and we simply don’t have enough Doppler data to find them. Astronomers need other techniques to map extrasolar planets at those large distances, and they then determine what the average planetary system looks like and whether ours is a typical solar system.

The three-year survey had a very strong and unequivocal result. It didn’t turn up even one giant extrasolar planet in the outer part of any of these nearby solar systems. According to Laird, “We certainly had the ability to detect super-Jupiters at ten A.U. and further out around young Sun-like stars.
The odds are extremely slight that planets larger than four or five Jupiter masses exist at distances greater than twenty A.U. from these stars, and so there is no planet oasis between twenty and a hundred A.U.” Another member of the team, grad student Eric Nielsen, who is also from Steward Observatory, said, “We achieved contrasts high enough to find these super-Jupiters but didn’t.”

Astronomers were surprised in the early days of planet hunting to find this population of massive super-Jupiters within the orbit of Mercury taking only a few days to orbit their planet. Now we know from this survey that there aren’t large numbers of giant planets lurking at large distances from their stars. We now have a much more complete picture of giant planet formation. The team used Laird Close’s novel Simultaneous Differential Imager, SDI, for observations made with the European Southern Observatory’s VLT, or Very Large Telescope in Chile, and they also used the six and a half meter MMT observatory in Mt. Hopkins here in Arizona. The new measurements show the power of imaging to weigh in on the demographics of extrasolar planets, and we can expect more results like this in the months and years to come.

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Planet-Hunters Hit Paydirt

First Week of June

This week astronomers were having their biannual professional meeting where they gather from all around the country and even around the world. This time it was in Honolulu, Hawaii. Yes, it is tough being an astronomer. Over a thousand people were there at the meeting, and one of the most news worthy items was the announcement of twenty-eight new planets outside our solar system, increasing to two hundred and thirty-six the total number of known exoplanets.

The discovery was announced by a pair of teams, the combined work of the California and Carnegie planet search team and the Anglo-Australian planet search team. These planets are among thirty-seven new sub-stellar objects, each orbiting a star but smaller than a star, discovered by those teams within the past year. Seven of the thirty-seven are confirmed brown dwarfs which are failed stars that nevertheless are much more massive than the largest Jupiter-size planets. Two others are borderline and could either be large gas giants or small brown dwarfs.

The California and Carnegie planet search team is headed by Geoff Marcy, professor of astronomy at UC Berkeley, and his colleagues Paul Butler and Debra Fischer, along with Steve Vogt who built the instrument they work with. Marcy and Butler are famous for the first discoveries of exoplanets back in 1995 along with a European team. The Anglo-Australian search team is headed by Chris Tinney from the University of New South Wales and Hugh Jones from the University of Hertfordshire. They have actually published their results over the past year, but this is the first time they’ve presented their entire year’s findings.

In addition to the thirty-seven sub-stellar objects, one of the team members singled out one particular exoplanet as extraordinarily rich. Circling the star Gliese 436, a red dwarf thirty light years distant from Earth, is an ice giant planet I talked about two weeks ago, calculated to be twenty-two Earth masses, slightly larger than Neptune. It’s been observed in transit of its star, which allows them to pin down the mass and calculate the planet’s radius and density. The planet turns out to be very similar to Jupiter. According to the team this planet must be fifty percent rock and about fifty percent water with small amounts of hydrogen and helium. People are going to follow up and try and get the atmospheric composition of this planet, according to the team.

Also among the twenty-eight exoplanets are four new multiple planet systems, plus three stars that probably contain a white dwarf as well as a planet. One of the team members said that at least a third of all stars known to have planets now have more than one, because smaller planets and outer planets of a star are hard to detect. He predicts the percentage will continue to rise as detection methods improve. He’s quoted as saying, “We’re just now getting to the point where if we were observing our own solar system from afar we would be seeing Jupiter.”

Three of the newly reported planets around large stars are between 1.5 and 1.9 times the mass of the Sun. The team focused on massive stars known as A and F stars which have masses between one and a half and two and a half times the mass of the Sun. Planets around these massive stars are normally very hard to detect because they typically rotate fast and have pulsating atmospheres, traits that can hide or mimic a signal from an orbiting star. However, the team discovered that cooler A stars, the sub-giants that have nearly completed hydrogen burning and stabilized for a short period of time, are quiet enough to make planet sized wobbles detectable. So far they have tracked down six previously discovered exoplanets around A stars, and by combining this set with three recently discovered exoplanets, they’re able to draw preliminary conclusions on the statistics.

Only one of the nine planets is within one astronomical unit of their host star, and none is within 0.8 AU of their host star, which is very different from the distribution around Sun-like stars. Even though short period planets are easier to detect, no such planets have been detected around the A stars. Based on the results of this search, they can calculate the odds of Jupiter-like giant planets. It’s about ten percent within two A.U. for stars between one and a half and two solar masses, versus only four percent for Sun-like stars with masses ranging from a half to one and a half solar masses, and only one percent for cool M stars less than about two-thirds the mass of the Sun.

These results are consistent with the popular core accretion model of planet formation. Large planets are expected to be more often observed around massive stars probably because these stars start out with more material in their disks during the early formation period.

Planet hunting has become an industry, and just to see how fast things are moving I went in the few days since the story was issued to the Extrasolar Planets Encyclopedia on the web at http://exoplanet.eu maintained by the European teams. Rather than two hundred and thirty-six, the census as of this past Friday was two hundred and forty-two planets, contained within a hundred and ninety-six planetary systems, including twenty-five multiple planet systems. Four planets were found by microlensing, four by imaging, there are four pulsar planets, and the vast majority have been detected with the Doppler or radial velocity technique. In the short time between this press release and me going to the web, another six planets had been found by other teams around the world. This field is moving quickly indeed.

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A Hot Ice Giant

Third Week of May 2007

With over two hundred exoplanets or planets around other stars known, it takes a pretty special exoplanet to rate a news story, but a very exciting planet was found last week, a hot ice giant around the star Gliese 436. This odd planet is the size of Neptune, and it’s made mostly of hot, solid water, it was discovered not too far from Earth, and it offers evidence that other planets may be covered with oceans. European astronomers reported the discovery this week.

The planet is called Gliese 436b. “It orbits around a cool, red star just thirty light years away,” said the team from the Geneva observatory led by Frederic Pont, an astronomer who helped make the discovery. He said, “It’s not a very welcoming planet.” It’s hot because it’s near its star and under high pressure because of its mass. In this strange state of matter the water is frozen by the pressure but it’s hot, hot enough to boil. It’s a very strange world. We’re used to seeing water changing conditions because of temperature, but in fact water can be solidified by pressure. “The planet is also likely to be blanketed by hydrogen,” said the researchers.

These conditions are hardly conducive to life, but if there is water there could be water on planets in other solar systems and then life as we know it. Not all the water on this big planet is likely to be frozen. Some of it could be liquid, and where there’s liquid water we believe there can be life. This planet is particular because it also passes in front of its parent star causing a mini-eclipse with an amount of light diminishing in proportion to its size. Knowing the size of the planet helps a lot with the interpretation because among the two hundred or so exoplanets known only a dozen or so have size variations measured by their eclipses.

From the size and the mass we can get the density, and the density of Gliese 436 suggests that it is made mostly of water. The researchers are not absolutely sure that the composition is water, but with this kind of density, and if you take the materials that usually make a planet, it’s very typical of water planets. What the researchers actually say in their report is that the mass and radius that we measure for Gliese 436b indicate that it is mainly composed of water ice. It is an ice giant planet like Uranus and Neptune rather than a small mass gas giant or a very heavy super-Earth. It’s also very close to its star which is an M dwarf or a cool red star, meaning a small star a hundred times less bright than the Sun and about half the Sun’s mass. Smaller stars are cooler and redder. They also live longer which is an interesting possibility as far as life’s concerned because this star may have lived a significant fraction of the age of the universe since the big bang, about fourteen billion years. The coolness of a red star is why water can persist, albeit in a hot and solid state on the planet. The astronomers estimate its temperature at five hundred and twenty degrees Kelvin which is two hundred and fifty degrees Celsius or five hundred and forty degrees Fahrenheit.

Gliese 436b is by far the closest, smallest, and least massive transiting planet detected so far. It’s only thirty-three light years away. Last month members of the same team said they had found the most Earth-like planet outside our solar system with balmy temperatures orbiting another red dwarf star called Gliese 581. The planet hunting game is “heating up,” and as we find planets like this, exotic water worlds, we have to wonder how strange life might be to inhabit such locales in the universe.

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Finding Clones of Home

Second Week of May 2007

NASA researchers have demonstrated that a space telescope rigged with the right equipment could actually photograph an Earth-like planet orbiting a nearby star. This is major news because as you probably know virtually all of the planets found so far are about the size of Jupiter, and virtually all of them have been found with the Doppler method, an indirect technique where the planet is revealed by the fact that it wobbles the star that it orbits. That wobble is detected as a periodic Doppler signal.

Imaging Earth-like planets is extremely difficult. They reflect less than a billionth of the light of their parent star, and they sit projected incredibly close to the star, when seen from far away. NASA researchers have made lab experiments as work in preparation for the Terrestrial Planet Finder, which is designed to hunt for an Earth twin that might harbor life. Trying to image an exoplanet is a daunting task because of the relatively dim glow and how much it’s overpowered by the bigger, brighter star. The challenge is often been compared to looking for a firefly next to a searchlight.

But two researchers at the JPL facility in Pasadena, California, have shown that a fairly simple coronagraph, an instrument used to mask a star’s glare, paired with an adjustable mirror could enable a space telescope to image a distant planet that’s ten billion times fainter than its central star. John Trauger, who’s the lead author of a paper in Nature magazine describes the work, “Our experiment demonstrates the suppression of glare extremely close to a star, clearing a field dark enough to allow us to see an Earth twin. This is a thousand times better than anything demonstrated previously.” The paper describes a new system called the High Contrast Imaging Testbed and how this technique could be used with a telescope in space to see exoplanets. The lab experiment used a laser as a simulated star with fainter copies of the laser star serving as the planets.

In the lab demonstration, the High Contrast and Imaging Testbed overcame two significant hurdles all telescopes face when trying to make images of Earth-like planets: diffracted and scattered light. When starlight hits the edge of a telescope’s primary mirror it becomes slightly disturbed in accordance with the wave nature of light, producing a pattern of rings or spikes around the major source of light in the focused image. This diffracted light could completely obscure any planets in the field of view. To address this problem Trauger and his colleagues fashioned a pair of masks for their system. The first, which resembles a blurry barcode, directly blocks most of the starlight while the second clears away the diffracted rings and spikes. The combination creates enough darkness to allow the light of any planets to shine through. “Mathematically, this coronagraph blocks the central star and its rings,” says Wesley Traub, another member of the team.

Scattered light presents an additional hurdle. Minor ripples on a telescope’s mirror produce speckles or faint copies of the star shifted to the side which can also hide planets. In fact they look planet-like, so speckles are often confused with planets. In the High Contrast Imaging Testbed a deformable mirror the size of a large coin limits scattered light with a surface that is altered slightly by computer controlled actuators. Many times a second this mirror compensates for the effects of minor imperfections in the telescope and instrument. Traub said, “This result is important because it points to the way to building a space telescope with the ability to detect Earth-like planets around nearby stars.”

Trauger and Traub plan to improve the suppression of speckles by a further factor of ten and extend the method to accommodate many wavelengths of light from red to blue simultaneously. I should also point out that this is not entirely a job for space alone. Here at the University of Arizona, we’re building a telescope called the Large Binocular Telescope. Its twin mirrors, giving it an effective aperture of 11.4 meters, work in concert. With an instrument called a nulling interferometer as its instrument, a similar suppression of the central starlight is possible. Deformable secondary mirrors correct for fluctuations in the Earth’s atmosphere, allowing this very large telescope, much larger than anything that can be launched into space, to separate a planet from the much brighter nearby star. Then, clever use of optics to overcome diffraction and scattered light, as with the JPL experiment, sufficiently blots out the central star to reveal the Earth-like planet.

Essentially there’s a race between ground-based facilities like the LBT and space-based projects like the Terrestrial Planet Finder. Both goals are the same, to find twins of the Earth, and it will be an exciting time indeed when astrobiologists find the first clone of our own planet and smear its feeble light into a spectrum to allow astronomers to detect chemical tracers of biology.

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The Most Earth-like Planet Yet

Fourth Week of April 2007

This week saw major news in astrobiology: the discovery of the most Earth-like planet yet found. European astronomers have discovered a planet only five times the mass of the Earth, a place where liquid water can almost certainly exist on its surface. It’s not exactly a system like Earth. The Sun wouldn’t burn brightly there. It would hang close, large and blood-red in the sky, glowing faintly like a charcoal ember, and it would probably never set if you lived on the sunny side of the planet. If you could have a birthday party it would happen every thirteen days; that’s how fast this new planet circles its Sun-like star. But watch that cake. It would weigh a lot more than it would on Earth. You might be able to keep your current clothes, however. The temperature in this alien setting will be a lot like the Earth. It’s not too hot and not too cold, and that just right temperature is the reason astronomers think this planet could conceivably house life outside our solar system.

It’s as close to Earth-size as telescopes have yet found, and both elements make it the first potentially habitable planet beside the Earth or Mars. There’s a lot that’s not known about this planet, but as planets go, and in the galaxy we live in, it’s a neighbor. It’s only a hundred and twenty trillion miles away. That’s about twenty light years. The red dwarf star that this planet circles is one of the hundred stars closest to the Earth. It was discovered by a European team at La Silla in Chile, the European Southern Observatory, which has built a special instrument to look for Doppler shifts to detect planets.

What they found is the planet circling the red dwarf star Gliese 581. Red dwarfs are low energy, tiny stars that give off dim, red light and last longer than stars like our Sun. Until recently astronomers didn’t even consider these stars as possible hosts for planets that might sustain life. The discovery of this new planet is sure to fuel future studies of planets orbiting similar stars since eighty percent of the stars near the Earth are red dwarfs. This new planet is about five times heavier than the Earth. Surface gravity there would be sixty percent stronger than on the Earth.

The discoverers are not certain if it’s rocky like the Earth or a frozen ice ball with liquid water on or near the surface. If it’s rocky like the Earth, which is accord with the prevailing theory, it has a diameter one and a half times the size of our planet, but if it’s an ice ball it could be even bigger. Based purely on theory, Gliese 581C could and should have an atmosphere, but what’s in that atmosphere is a mystery. If it’s too thick or greenhouse heating occurs, the planet’s surface might be too hot for life. However, the research team believes that the average temperature is somewhere between thirty-two and a hundred degrees Fahrenheit, and that set off celebrations amongst astronomers, because until now every one of the two hundred and twenty planets astronomers have found outside the solar system have had the Goldilocks problem. They’ve been too hot, too cold, or just plain too big and gaseous like Jupiter in our solar system. This new planet seems just right.

Besides having the right temperature, the new planet is probably full of liquid water as hypothesized by Stephane Udry, the discovery team’s lead author and an astronomer from the University of Geneva, but that’s based solely on theory about how planets form and not on any direct evidence. Co-author Xavier Delfosse of Grenoble Observatory said, “Liquid water is critical to life as we know it.” Due to its temperature and relative proximity, this planet will most probably be a very important target of future space missions dedicated to the search for extraterrestrial life. Said Udry, “On the treasure map of the universe, one would be tempted to mark this planet with an X.” Other astronomers cautioned it’s a little too early to tell whether there’s water.

The new planet’s star system is only twenty and a half light years away, making it a very close companion to the Earth, but the star is so dim you can’t see it without a telescope, it’s somewhere in the constellation Libra which is currently low in the southeastern sky in the United States in the mid-evening. “Even so,” noted Steve Maran, the press officer for the American Astronomical Society, “we have no idea of how to get to these places in a human lifetime.” But the view if you could? The planet is fourteen times closer to the star it orbits than we are to the Sun. Udry has figures the red dwarf star would hang in the sky at a size twenty times larger than our Moon, and it’s likely, but still not known, that the planet doesn’t rotate so one side would always be sunlit and the other dark.

This is an exceptional discovery because it’s going to trigger searches for planets around the multitude of dim, red stars in our own galaxy. If habitable zones are extended to stars quite different from the Sun, much dimmer, much less luminous, then the number of potential habitable places in the galaxy could go up by a factor of ten, which is exciting news indeed for astrobiology.

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Water and Foliage on Exoplanets

Second Week of April 2007

This week I want to talk about the two colors of extrasolar planets: blue and green. For blue, I’m speaking metaphorically or even poetically because the blue refers to water. Water is of course colorless and only appears blue in its liquid or solid form due to scattering, but blue and the water that implies life has always been sought in extrasolar planets. Over two hundred are known, and a few weeks ago for the first time water was identified in the atmosphere of one of them through a combination of previously published Hubble Space Telescope observations and new theoretical models.

Water is one of the most abundant molecules in the universe, so it’s not in fact surprising that it’s present in the atmosphere of extrasolar planets. But almost all of the two hundred plus planets known have been found by the Doppler method, and the Doppler method, whereby the planet reveals itself in its tug on the parent star and a periodic wobble of that star, gives very little information other than the mass of the planet. To learn more we need the very particular geometries of eclipsing solar planets. That is, we have to depend on the tiny fraction of the planets where the plane of the orbit happens to lie exactly along the line of sight to the Earth so that once every orbit the planet passes between us and its star, and once every orbit it passes on the far side of its star.

One of the extrasolar planets, a super-Jupiter with a very fast orbit and a very close massive planet, HD209458, displays such eclipses. Every three and a half days the planet passes in front of the star. During one of these observations, the Hubble Space Telescope measured spectra at optical and infrared wavelengths. When the planet passes in front of the star, its atmosphere blocks a different amount of the starlight at different wavelengths, and in particular absorption by water in the atmosphere makes the planet appear larger across a special part of the infrared spectrum compared to wavelengths in the visible spectrum. A careful analysis of this data and a comparison to new models by researchers at Lowell Observatory has produced evidence for water absorption in this planet, a hundred and fifty light years from the Earth.

It’s an exciting discovery but fair to say that it’s disputed by other astronomers. Theoretical modeling is important in the interpretation, and other theorists have produced conflicting results. This one is not yet nailed. What about the green of extrasolar planets? Well that’s the most exciting of all, the ability to detect microbial life and in particular vegetation, plants, on distant worlds. Research done at the Virtual Planetary Lab, a part of the California Institute of Technology’s NASA astrobiology initiative, has shown that if life exists on other planets, and it’s plant life, it may not be predominantly green. Scientists found that the predominant color of the foliage on Earth could be a range of other colors depending on the star that the planet orbits and the composition of the atmosphere.

Other planets may have foliage that’s yellow, orange, or red. It all depends on the color of the star. Determining the range of colors is important because scientists need to know what to look for when they begin gathering spectra of light from distant Earth clones. Lead author Nancy Kiang, a biometeorologist at NASA’s Goddard Institute of Space Studies and a visitor at the Spitzer Scientist Center, says, “The dominant color of photosynthesis could be yellow, or orange, or maybe red. I think it’s unlikely that anything will be blue, and of course green plants are also a possibility since that’s what we have here.” One of the most fun parts of this research is how interdisciplinary it is. These results could not be derived unless planetary scientists, atmospheric scientists, biologists, and others had pooled their expertise in modeling the possible spectra of light available to plants on Earth-like planets around other stars.

Vicki Meadows is the leader of this group, and she knows exactly how much expertise has to come to the table. “The study requires data about everything from the type of photons given off by a main-sequence star in a particular stage of its life, to the depth of water that an aqueous plant might prefer,” and, “a huge variety of information.” Meadows says, “No single astronomer or biologist or atmospheric scientist could have attacked this problem individually.” The researchers focused on the way plants use light for energy to produce sugar, which is pretty much the definition of photosynthesis because photosynthetic pigments must be adapted to the available light spectrum. The available light at a planet’s surface is a result both of the light from the parent star and filtering effects of the gas in the atmosphere.

For example, ozone on the Earth absorbs ultraviolet light, so not much of that reaches the Earth’s surface. Kiang said, “It turns out that the spectrum of the number of particles of light is what is important, and on Earth there are more particles in the red,” which, “could explain why plants [. . .] on Earth are mainly green.” On Earth, plants absorb the blue light because it is energetic and red light because its photons are plentiful. There is more than enough energy from the blue and red in sunlight, so plants do not really need more. Therefore, they reflect away relatively more green light, which is why they appear green to us. But on other worlds with other atmospheric compositions, things could be quite different, and so we’re starting to learn what the range of colors of planets with plant life might look like. That’s an exciting new advance in astrobiology.

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