As the hunt for Earth-size planets around other stars ratchets up, some researchers are already investigating how to test those planets for habitability once they are found.

A team of U.S. and Australian scientists reports successfully testing an approach that could be used to look for water on so-called extrasolar planets, or exoplanets. The researchers tried out their technique, described this week in a paper submitted to arxiv.org to be published in Astrobiology, on the only Earth-like planet we know of: Earth.

Two teams of astronomers made headlines in November after announcing they had photographed planets orbiting regular stars other than our own sun. (Such bodies are known as extrasolar planets, or exoplanets.) One of those planets, Fomalhaut b, the companion to a star called Fomalhaut some 25 light-years away, was spotted by the Hubble Space Telescope; a star known as HR 8799, nearly 130 light-years distant, was found by ground-based observations to harbor a system of at least three planets.

Now reprocessed images taken by Hubble in October 1998 show that the space-based observatory had picked up the signal of the outermost of HR 8799's planets (white dot at lower right in image) 10 years prior to that announcement. Astronomer David Lafrenière, a postdoctoral fellow at the University of Toronto and a member of the team that found the planets around HR 8799 last year, led the archival work. In a statement, he credited an improved image-processing technique that he and colleagues developed for uncovering the hidden planet, saying it can now be used "to see planets that are one-tenth the brightness of what could be detected before with Hubble."

Late tomorrow night, NASA's Kepler spacecraft will—conditions permitting—lift off from Cape Canaveral Air Force Station in Florida on its unprecedented mission to find habitable, Earth-like planets around distant stars.

The $600-million venture will train Kepler's photometer on a group of 100,000 stars several hundred to a few thousand light-years away and track them for more than three years. As the planets believed to reside there pass between their stars and Kepler's detector, the spacecraft will register a slight dip in stellar brightness. Over time these dips can be used to compile a profile of the planetary systems in Kepler's view, which, according to prevailing models of planetary formation, should include several Earth-like planets. Once astronomers know how common Earths are and where in the galaxy they can be found, follow-up missions will be able to refine their searches for extraterrestrial life.

The exoplanet express just keeps on rolling: The European Space Agency (ESA) today announced the discovery of the smallest known planet orbiting a normal star other than the sun. The newly discovered extrasolar planet, or exoplanet, which appears to be terrestrial (that is, not a gas giant like Jupiter), has a radius less than twice that of Earth.

The exoplanet has been dubbed COROT-Exo-7b after the European satellite COROT, which located it. The space telescope revealed the exoplanet by monitoring how the light from its parent star dimmed as the planet transited, or passed in front of the star during orbit (see brightness graph, not to scale, at left). Most of the 300-plus exoplanets so far discovered have been found using this method or by observing the wobbling of stars induced by the gravitational pull of orbiting bodies.

An international consortium of researchers a few months ago reported what appeared to be the smallest planet yet detected orbiting a star other than our own sun. Such objects are known as extrasolar planets, or exoplanets, and most of the 300-plus so far discovered are quite large in size compared to Earth. But a new estimate of the size of the smallest exoplanet, known as MOA-2007-BLG-192Lb, reportedly brings its mass down even further—and even closer to that of Earth. (An even smaller object had been observed in 1992, but it orbits a stellar remnant known as a pulsar.)

Astrophysicist Jean-Philippe Beaulieu of the Institute of Astrophysics in Paris, a member of the team that discovered MOA-2007-BLG-192Lb, presented his new estimate at a recent meeting of the Royal Astronomical Society in London, according to the New Scientist. Whereas the original estimate for the exoplanet's mass was roughly three Earths, the new number is a mere 1.4 Earth-masses. (Beaulieu could not be reached for comment today.)

It was front-page news when astronomer Paul Kalas of the University of California, Berkeley, and his team produced the first photographic evidence of a planet orbiting another star last month. (Another team, publishing simultaneously, announced similar results around a different star.) But at least one person in the field was not surprised: astronomer Alice Quillen of the University of Rochester had predicted the existence of just such a planet, in just such an orbit, in the Monthly Notices of the Royal Astronomical Society: Letters in October 2006.

Two years ago, Quillen examined the debris disk around a star 25 light-years away known as Fomalhaut. She hypothesized that the features of the disk implied that there ought to be a planet, whose mass lay between that of Neptune and Saturn, orbiting nearby, some 119 astronomical units (AU) from Fomalhaut. (An astronomical unit is roughly the distance between Earth and the sun.)

A poorly kept secret is now official: the Hubble Space Telescope has discovered carbon dioxide (CO₂) in the atmosphere of a planet outside our solar system. That's a first in the study of extrasolar planets, or exoplanets, which have been quite the hot topic this year.

The exoplanet HD 189733 b, roughly the mass of Jupiter, orbits a star 63 light-years away in extremely close company. Although the planet can't be seen directly, scientists used Hubble data to analyze its atmospheric composition and turn up CO₂ as well as carbon monoxide (CO). They did this by comparing the light spectrum from the star with that from the star and planet combined, as the planet passes in front of its star.

Although HD 189733 b is way too steamy for life as we know it—roughly 1,950 degrees Fahrenheit (1,065 degrees Celsius) by one estimate—the finding, leaked to media outlets two weeks ago, shows that techniques exist to find markers of life on other planets. (The paper has been submitted to Astrophysical Journal Letters.)

On the heels of the first photographs of planets orbiting other stars comes another first for so-called extrasolar planets: an atmosphere containing carbon dioxide (CO₂). Nature News and Science News report that a forthcoming journal article will detail the discovery of CO₂ around HD 189733 b, a planet roughly equivalent to Jupiter in mass that orbits a star some 63 light-years away.

HD 189733 b, discovered in 2005, has already yielded other exoplanet milestones: it was the first found to host an atmosphere containing methane and was also among the first found to harbor water vapor. All of these discoveries have been made without seeing the planet in the conventional sense: to ascertain a planet's traits, the light spectrum of the parent star is compared with the star's emission as the planet passes in front of it. In the latest finding, the data came from the Hubble Space Telescope.