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.

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.)

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.

NASA’s Cassini orbiter is sweeping past Saturn’s icy moon Enceladus today to photograph geyser-like eruptions from the Southern Hemisphere. The hope is to find signs of the building blocks of life: NASA has pegged Enceladus as one of the most likely places in the Solar System (along with Mars and another of Saturn’s moons, Titan) to be able to support extraterrestrial life.

On a flyby earlier this year, the Cassini spacecraft detected organic chemicals like methane and propane in the contents spewing out of the moon’s interior. Such compounds have also been found in comets’ tails and just recently on Titan.