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From SPIE Professional April 2006

Outer, outer space

Astronomers the world round peek further out into the universe - and glean more than ever from their search.
31 April 2006, SPIE Newsroom. DOI: 10.1117/2.4200604.02
British writer Eden Phillpotts once said, "The universe is full of magical things, patiently waiting for our wits to grow sharper." Astronomers know that to discover the outer reaches of our known universe, our imaging must grow sharper as well.
At The Search for Extra-Solar Planets invited session during the Astronomical Telescopes and Instrumentation symposium (see sidebar), several researchers will describe the current search for exoplanets, the tools and missions used to do so, and even the search for life on these planets.

Roberto GilmozziWesley TraubSara Seager
Not long ago the search for life on other planets was considered the stuff of dreams. Now it's an area of scientific research experiencing rapid advances. More exoplanets are discovered all the time. Now the added challenge is extracting information from the mysterious planets.
"Most of these planets are not able to harbor life as we know it since they are gas giants with no surface," says Sara Seager, staff scientist at the Carnegie Institution of Washington (Washington, DC). "Because even the most ambitious technical projects would only be able to see the planet in a global sense -- not spatially resolved -- the search for life on exoplanets is more difficult than just finding the kind of exoplanets we think could harbor life. Nevertheless, I have no doubt that Earth-mass planets are out there and when today's technological advances mature, the search for life or signs of habitability will become very prominent indeed."
That's just what Wesley Traub at NASA's Jet Propulsion Laboratory (Pasadena, CA) is working on with the space-based Terrestrial Planet Finder Coronagraph (TPF-C) and Terrestrial Planet Finder Interferometer (TPF-I) missions. The two missions allow the observation of planets in the visible and mid-infrared spectral regions in order to more fully characterize the planets.
"In particular, in the visible and near-infrared, TPF-C can detect oxygen, water, ozone, the red edge of vegetation, and the total column of air on a planet like the present Earth," explains Traub. "If we find a planet like the early Earth, TPF-C will be able to detect carbon dioxide and methane, both likely to have been abundant once. TPF-C will also be able to detect variations of these quantities, leading to other inferred properties including the length of day, weather activity, seasons, and relative areas of land and ocean.
"In the mid-infrared, TPF-I can detect carbon dioxide, ozone, water, methane, effective temperature, diameter, and effective albedo on a planet like the present Earth. On a planet with more nitrous oxide than Earth, it could detect that as well."
Traub touches on an important tool to characterize exoplanets -- our own planet's history.
"Astronomers have been studying 'Earth as an exoplanet' by considering what Earth would look like as viewed by a very distant space telescope," says Seager. "Interestingly, we don't just have current Earth as an example of a habitable exoplanet, but we have Earth's past appearance as well. Earth's past appearance includes very different continental arrangements, episodes of widespread glaciation and possibly a time when methane was a strong atmospheric absorber."
Ground-based observatories are the Earth-bound cousins of space missions such as TPF-C and TPF-I. Breaking advances in ground-based imaging are increasingly contributing to the search for, and description of, exoplanets.
"There is a method called differential imaging that is used to search for planets around nearby stars that is very promising," explains Roberto Gilmozzi, European Southern Observatory (ESO; Garching, Germany). "The contrast between a possible planet and a star is enhanced by obtaining simultaneous images at slightly different wavelengths chosen so that some spectral feature in the planet makes it brighter in one of the images while the star maintains the same brightness. Subtracting the images cancels the light from the star and makes the planet stand out."
Gilmozzi says another development is a new take on the use of adaptive optics to correct for the light pollution from neighboring brighter stars.
"Several observatories, including ESO, are working on new instrumentation using 'extreme adaptive optics' to achieve even higher contrast images. Extreme adaptive optics produces a much better correction than what we have today. For example, the Planet Finder being designed for the VLT [Very Large Telescope] will use this kind of adaptive optics, correcting more of the light, together with techniques like differential imaging, or its equivalent using the polarization of light, to be able to detect planets up to 10 million times fainter than their parent star."
As will be evidenced at the invited session, both ground-based and space-based imaging are vital to the exoplanet search and both have their unique advantages.
"In space the turbulent atmosphere is absent, so atmospheric speckles in the focal plane are absent, and it is possible to image a faint planet near a bright star," says Traub."[A] second advantage is that from space it is much easier to detect the absorption features of an Earth-like planet, because the absorption of the Earth's atmosphere is no longer blocking part of the signal."
On the other hand, an advantage of ground-based observatories lies in their telescopes' larger diameters coupled with adaptive optics.
Gilmozzi says, "The advent in the next decade of extremely large telescopes with diameters of 30 m or more will achieve much higher resolutions than telescopes in space, really opening up the field of extra-solar planet imaging, possibly even of Earth-like planets."
"Both ground-based and space-based facilities will play an increasing role in the search and characterization of exoplanets," says Seager. "New ground-based techniques are maturing, finding new kinds of exoplanets, while space is essential for characterizing planets by detecting their atmospheres and especially for the search for low-mass exoplanets and their characterization."
The bottom line, though, is that the hunt for exoplanets is a burgeoning and exciting field.
"I believe that the search for Earth-like planets, and for signs of life on them, is the most important project of my lifetime, or of three lifetimes if that were possible," says Traub."We are privileged to live in an age when we can ask such questions, and expect to get answers.And as scientists and engineers of SPIE, we are especially privileged to be the ones who have it within our grasp to build the telescopes that can answer such fundamental questions."

Erin M. Schadt
SPIE Professional Managing Editor