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Exotic Earths

David Lamb, NASA Astrobiology Institute

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Summary: New astronomical techniques are vastly increasing what we know about extrasolar planets. Scientists have already found a large number of Jupiter sized bodies, but what about Earth-like planets in other solar systems?

New astronomical techniques are vastly increasing what we know about extrasolar planets. Scientists have already found a large number of Jupiter sized bodies, but what about Earth-like planets in other solar systems?

At the heart of astrobiology lie fundamental questions regarding the nature and prevalence of life. At the very minimum, life as we know it requires liquid water, a constant supply of energy, and specific elements to support its organic systems. But perhaps even more important than this laundry list of prerequisites, organisms require a home—a place that can constantly support and supply all that is necessary to keep them thriving in their environment.
Currently, Earth is the only place we know with unequivocal evidence for life; however, scientists have many good reasons to expect we will find life elsewhere. Much of the current research in astrobiology is focused around finding Earth-like conditions on nearby planetary bodies such as Mars or Jupiter’s moon Europa. Nevertheless, there is no reason to believe that life would be confined to our solar system, and it is partly due to technological limitations that have rendered a local search to be a good starting point. But has technology started to reach the point where astrobiologists can elucidate questions about finding life among other stars?

For many years, astronomers speculated about he possibilities of planets beyond our solar system. However, it wasn’t until 1995 that astronomers were actually able to detect these planets (http://physicsweb.org/article/world/13/3/7). Extrasolar planets are found by measuring the gravitational affect they have on their parent star (this is called the radial velocity technique). With early efforts, only massive planets were able to be detected since it would take a large body (akin to the gas giant planets in our own solar system) to noticeably influence a much larger star. But newer searches have been more direct. Just recently, astronomers from the Harvard-Smithsonian Center for Astrophysics detected an extrasolar planet by measuring the dimming of a star as a planet passed in front of it (see “New world of Iron Rain”). This is called the transit method since it measures how much a transiting planet obscures starlight. The dimming of light caused by a transit is very small; still, astronomers are able to measure this change and this new technique greatly extends the capabilities for monitoring star systems.

Although discoveries like these are inspiring as they spark our imaginations of what’s out there, how much can they really say about the possibilities of extrasolar life? The extrasolar planets being catalogued are not the kind of places you’d look for signs of organisms. Many of these worlds are Jupiter-like, very different from the conditions on Earth. Even though there is a possibility that many, if not most, of these planets have moons that could possibly support life, there is no current method to detect extrasolar moons. Moreover, even if Jupiter sized planets had moons, the planets would still need to be in a large enough orbit so that the moons would not be baked by their local sun. The planet recently found with the transit method, OGLE-TR-56b, orbits at a distance of only 4 stellar radii. At that distance, the surface temperature would reach around 3,000 degrees Fahrenheit!

In order to know more about the possibilities for extrasolar life, researchers need to detect and verify Earth-like planets. And fortunately, NASA already has missions underway to do just that. The first of these, the Kepler Mission (led by William Borucki of NASA Ames) is specifically designed to make sensitive transit measurements for Earth sized planets. Kepler will be able to measure changes in stellar brightness on the order of 100 parts per million! In order to achieve this precision, measurements will be spaceborne. The orbiting telescope will also do away with day-night and seasonal variations that interrupt ground-based observations. Kepler will be simultaneously monitoring 100,000 stars for a period of four years. Its launch is currently scheduled for 2006.

Although the Kepler mission will give us a better understanding of the number of Earth sized planets, it will not be able to tell us about their chemistry. Scientists predict that if an Earth-size planet harbors life, it will likely have similar surface and atmospheric characteristics. Earth’s atmosphere is out of chemical equilibrium. If it were not, life would not be possible for many advanced organisms. Photosynthesizing plants and bacteria keep pumping oxygen gas into our atmosphere. If somehow this process stopped, then Earth would eventually go the way of Mars or Venus, i.e. there would be only trace amounts of oxygen and a preponderance of carbon dioxide. Similarly, unlike Mars and Venus, Earth has a component of methane in its atmosphere. This too is due to active processes of Earth’s biota. Trademarks such as oxygen and methane content in planetary atmospheres would tell astronomers that an extrasolar planet likely had similar processes occurring, which would mean that these places were excellent candidates for finding life.

With ideas of extrasolar atmospheric chemistry in mind, NASA is currently developing the Terrestrial Planet Finder (TPF). TPF’s onboard spectrometer will allow scientists to detect relative amounts of key life supporting gases. Spectral characterization of extrasolar planets will be the next big step in looking for life beyond our solar system. However, TPF is still on the drawing board and its final design is awaiting near term technology development. The final architecture of the Terrestrial Planet finder is slated to be selected in 2006. Log onto JPL’s Planet Quest website for continuing developments: http://planetquest.jpl.nasa.gov/.

Even though direct detection of exotic Earth’s might be years away, researchers are seeking to understand more about them in the nearer term. This is possible because of advanced modeling techniques that will be occurring in the Virtual Planetary Laboratory (VPL). Vikki Meadows, the Principal Investigator for NAI’s JPL 2 Team, will be leading 28 researchers in the simulation of potential planetary environments. Currently, scientists have only one model of how a habitable planet works: our own home. But there is no reason to think that this is the only model for a life-sustaining planet: there could be an entire range of terrestrial planets that can support organisms. The VPL will be utilizing some of the best supercomputers available; as Meadows explains, “We’re trying to build a terrestrial planet inside a computer…This will help us determine what the signatures of life on an extrasolar planet will look like, once we have the technology to study them.” For more on the Virtual Planetary Laboratory, check out Building Planets in Cyberspace.

As the list of extrasolar planets continue to grow and scientists refine their techniques for detecting them, there is a mounting excitement for finding life-sustaining conditions elsewhere. It seems that every time astronomers enhance their accuracy in detection methods, they find only more and more planets. Some day in the near future it could be that when astrobiologists gaze in their telescopes to distant star systems, they will find something that looks like a little closer to home.

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