This article is reproduced with the permission of New Scientist for exclusive use by Nova users.

Look of life...
12 July 2003
From New Scientist Print Edition.
Betsy Mason

Enlarge Is life on Earth unique?

Within a decade, both NASA and the European Space Agency (ESA) will launch a new generation of spacecraft. These will be the first that are capable of spotting tiny planets, anywhere in the galaxy, that may support life.

It is an onerous task only made possible by stretching cutting-edge instruments to their limits. However, even at this early stage, there is already a problem. The missions are, of course, closely tied to our concept of life. But, despite all we know about the chemistry of life on Earth, researchers disagree about exactly what they should be looking for. Are the signs of life best spotted using visible light? Or is an infrared telescope the best tool for the job? ESA and NASA are already on diverging paths.

At least both agencies are clear about one thing: the need to do more than simply notch up a tally of extrasolar planets, without knowing anything about the chemistry of these other worlds. Today's planet hunters look for the gravitational effect planets have on their host star: of the 102 extrasolar planets known, the smallest are more than 50 times the size of Earth and very little is known about what these worlds are like. These new missions will change that: they will study the planets' atmospheres for signs of life.

ESA is planning a flotilla of six spacecraft flying in formation, each carrying telescopes with 2-metre mirrors, to capture infrared light. NASA may join the project, called Darwin, but is also considering an 8-metre visible light telescope called Terrestrial Planet Finder (TPF) that will dwarf Hubble and the next-generation James Webb space telescopes. In both cases, Earth-like planets will be at the absolute limit of the instruments' resolution, amounting to a single pixel, a pale blue dot.

Both Darwin and TPF need a way to subtract the light of the host stars from the image. Darwin's infrared concept has an advantage here: stars like our sun shine a billion times more brightly than an Earth-like planet in the visible part of the spectrum, but only a million times more brightly in the infrared. This means less processing is needed to obtain an infrared image in which the planet is not swamped by the host star's light. But a longer wavelength means a telescope has to be effectively much larger to resolve the light, and this is why so many spacecraft are needed.

Instead of building a 100-metre mirror, which isn't yet possible, the six craft will fly in a 100-metre formation to deliver the same imaging power. A seventh craft will carry a hub that combines images of each star system in such a way that the infrared light waves they collect from the star will be cancelled out, and those from the nearby planet added together to become brighter. An eighth communications satellite will relay the pictures back to Earth. "Technologically this may be the most challenging thing ever attempted by any of the space agencies," says Malcolm Fridlund, project scientist for ESA's Darwin project in Noordwijk, the Netherlands, "but it's feasible." A pilot ESA mission planned for 2006 will test the principle with just two craft flying in formation, although it will only be sensitive enough to image extrasolar planets that are too large to be Earth-like.

NASA's TPF teams are also working on an infrared design, but their preferred technology is a single visible light telescope. This mission would involve just one spacecraft carrying an 8 to 10-metre mirror and operating with an opaque black disc, called a coronagraph, designed to block the light from the host star. The visible pictures will be more precise and of better quality than infrared images, because of visible light's shorter wavelength.

ESA scientists insist the infrared portion of the spectrum will be far more revealing about the temperature and size of the distant planet, as well as the presence of methane, which dominated Earth's atmospheric history for so long. They think NASA will abandon the visible light design. "It looks most probable to me that we will both end up with the interferometer," says Fridlund. But a final decision won't be made until 2005.

To aid that decision, NASA has commissioned research to see what our own planet might look like on an image taken by a struggling telescope from halfway across the galaxy. This is not simply a matter of fuzzing up one of the many images of the Earth available from orbiting satellites. Satellites only look at a small portion of the Earth at once. From afar, a telescope will see a whole planet averaged together in a single dot.

One approach is to add together thousands of spacecraft views. But even then the result could be biased because spacecraft are either looking straight down at the region of the Earth immediately below, or up at the horizon, the edge of the atmosphere. So light from some regions on Earth is under-represented in available images.

Wesley Traub at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Neville Woolf of the University of Arizona in Tucson have found a way around this problem. They have studied the light from Earth that is reflected by dark portions of the crescent moon, capturing this "earthshine" using the 2.3-metre telescope at Steward Observatory on Kitt Peak in Arizona.

Any features are automatically averaged out by the rough nature of the moon's surface. The result was an averaged spectrum that did not match what they expected from looking at available visual images. "The sort of surprising thing is that there was such a strong blue signal," says Traub. The thick gas atmosphere of Earth scatters short-wavelength light back out to space, creating a striking increase in the visible part of the spectrum towards the deep blue, and ultraviolet.

The effect raises the possibility of measuring the thickness of any planet's atmosphere by looking at the steepness of the increase in signal moving towards the blue end of the spectrum. The rate of change should be proportional to the total number of gas molecules in the atmosphere that are scattering incoming light. On Earth, most of this is due to nitrogen gas molecules. To host life at all, a rocky planet will need a thick atmosphere to prevent liquid water being sucked off into space.

On top of the bluey spectrum, Traub and Woolf saw spectral features that correspond to light being absorbed by clouds, oxygen, carbon dioxide and water, all of which are indicators of habitability. But making the link from habitability to habitation is the key. "If you could see large amounts of molecular oxygen, you would feel pretty confident that you were looking at a planet where life was generating this oxygen," says Traub. For a telescope working at the limits of its ability, the easiest and clearest signature to look for is an ozone line in the ultraviolet. "It is deeper and narrower," says Traub. "From a spectroscopic point of view, it is the winner by far."

Ozone is formed when ultraviolet light hits oxygen molecules, so its presence points directly to the presence of oxygen. An infrared-only telescope would not be able to see this ozone line - just the sort of thing that ESA and NASA need to take into account in designing their missions. There are other ozone lines in the infrared part of the spectrum, but their appearance is not sensitive to the amount of ozone in an atmosphere, only to its presence.

But relying on a high level of oxygen as the only sign of life could be a potential trap. On a planet with a runaway greenhouse effect, like Venus, ultraviolet light can split water molecules into hydrogen and oxygen, and the lighter hydrogen could be sucked off into space before reacting with the oxygen again. This could also happen on a very frozen planet, a kind of "snowball Earth" where minerals in the crust that would usually react with oxygen become frozen under a layer of ice. If the planet is too small to support volcanic activity, or hold hydrogen, there will be no reducing gases in the atmosphere to react with the oxygen.

Both of these scenarios involve temperature extremes and so the next step in establishing the biology of an apparently habitable planet will come from its temperature. The temperature is revealed by the overall shape of the infrared spectrum: a cooler planet emits more light at longer wavelengths. The overall brightness reveals the surface area of the planet, and therefore its size. As the visible spectrum does not give this information, many researchers think this is a good reason to go for an infrared telescope.

Traub, however, suggests a way to find the temperature of a planet using only the visible light spectrum. The brightness of a planet reveals how reflective it is. Radiation from the host star that is not reflected goes into the atmosphere and warms up the planet. One hitch is that this only gives information about the uppermost atmosphere. On Earth this is below the freezing point of water. To find the surface temperature, researchers need to make assumptions about how the atmosphere of the planet changes below the top layer. The cloudier the planet, the harder this will be.

To clinch the case for life, researchers such as James Kasting at Pennsylvania State University in University Park, who is creating a list of biomarkers for NASA, say it is important to find any oxygen in combination with a reducing gas such as methane. Because methane and oxygen naturally react together to form carbon dioxide and water, their coexistence shows they are being continually generated. This would point to the presence of some kind of oxygen-producing photosynthetic life.

Methane is also an important indicator of life in its own right, especially on the early Earth. Geological records show that 2 billion years ago the atmosphere contained a thousand times as much methane as it does today. Geologists assume the methane was produced at least partly by bacteria, and accumulated because not enough oxygen was yet present to react with the methane and remove it from the atmosphere. This is yet another reason not to rely on oxygen as the only signal that a planet is inhabited, says Kasting.

Detecting methane in the infrared is easy, but in the visible part of the spectrum the amount of methane available on Earth today would not show up.

Traub and Woolf's earthshine experiment also found faint evidence of light reflected by land plants containing chlorophyll. Although plants reflect green visible light, they also reflect at long red wavelengths that the human eye cannot detect. Seeing this would be good evidence that a planet was extremely similar to Earth, but there could be some otherwise inexplicable effects in the spectrum of a planet that could point to innovative use of the local star's energy by life forms.

If such spectral anomalies appear, they could vary as the planet rotates, indicating the presence of more or less inhabited regions. So far, the earthshine measurements from Kitt Peak are not sensitive enough to reveal a difference between the plant-laden continents of Europe and Asia as opposed to the blue of the Pacific Ocean. But Traub and Woolf are applying to NASA for funds to build a telescope at the South Pole. Traub calculates the effect of rotation should be visible in images taken there, because the thinner atmosphere makes for better moon viewing.

By the time NASA has to make a decision on the telescopes, both missions also hope to have narrowed down the list of candidate star systems to a few priority systems. None of the known giant extrasolar planets can share its system with terrestrial planets in the habitable zone where liquid water could exist, because calculations show gravitational forces would tear small planets in those regions apart. But there are hundreds of nearby solar-type stars where this may not be the case. A NASA telescope launching in 2007 called Kepler will monitor 100,000 main sequence stars, looking for momentary dimming as Earth-like planets pass in front. Star system formation calculations suggest this should produce around 50 rocky planets to be imaged directly by the later missions.

But even assuming life is quite common among solar-type stars, both missions face a major worry: time. Even on a perfect planet, the window of opportunity for life isn't that big. "Our planet is not going to be very Earth-like pretty soon and it wasn't very Earth-like not very long ago," says palaeontologist Peter Ward of the University of Washington in Seattle, author of The Life and Death of Planet Earth. The planet is 4.5 billion years old now, but in around 500 million years, the sun will begin to swell up, becoming too hot and bright for the plants that exist today. The high levels of oxygen that plants and multicellular creatures began to pump out about 600 million years ago will begin to fall. This means a detectable oxygen signature will last around a billion years on Earth, less than a tenth of the planet's 12-billion-year lifetime.

But even within this constraint, the new missions will be the first to test our understanding of the factors that lead to life on Earth. Whether Darwin and the Terrestrial Planet Finder find life or not, the samples of hundreds of other atmospheres that they will collect will launch a new field of comparative planetary science. No longer will planetary researchers be confined to our biased sample of our own inner star system. It's almost a guaranteed success: with these missions will come the first real evidence about the uniqueness of life as we know it.

Betsy Mason is a science writer in Connecticut

From issue 2403 of New Scientist magazine, 12 July 2003, page 28

For the latest from New Scientiist visit www.newscientist.com