* Astronomy

The possibility of discovering a planet that is small, cool, rocky, orbiting a sunlike star and able to host life - an Earth twin, in other words - has made the search for planets outside of our solar system, or exoplanets, one of the hottest research areas in physical science. This three-part series explores MIT researchers' roles in the quest to find an Earth twin and the effort to make sense of the 500 exoplanets that have been discovered since 1995.
In September, researchers announced the discovery of Gliese 581g, a rocky planet with a mass that is just three to four times that of Earth. If the discovery is confirmed with independent data, it could be the closest that planetary scientists have come to finding a planet outside the solar system that resembles our own. Although other planets with nearly the same mass as Earth have been discovered, Gliese 581g is the smallest planet that is also in the "Goldilocks zone," or at a distance from its host star to make the planet's temperature cool enough for liquid water to exist on its surface.
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Watery, rocky planets are commonplace in the Milky Way

Astronomers have found evidence that rocky planets are commonplace in the Milky Way, or the Galaxy.
Dr. Jay Farihi, Leicester University scientist and lead author, surveyed white dwarfs, the compact remnants of stars that were once like our Sun, and found that many show signs of contamination by heavier elements and possibly even water, improving the prospects for extraterrestrial life.
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Rocky planets 'are commonplace' in our Galaxy

An international team of astronomers have discovered compelling evidence that rocky planets are commonplace in our Galaxy. Leicester University scientist and lead researcher Dr Jay Farihi surveyed white dwarfs, the compact remnants of stars that were once like our Sun, and found that many show signs of contamination by heavier elements and possibly even water, improving the prospects for extraterrestrial life. On Tuesday 13th April Dr Farihi will present his results at the RAS National Astronomy Meeting (NAM 2010) in Glasgow.
White dwarf stars are the endpoint of stellar evolution for the vast majority (>90%) of all stars in the Milky Way, including our Sun.  Because they should have essentially pure hydrogen or pure helium atmospheres, if heavier elements (in astronomy described as metals, examples including calcium, magnesium and iron) are found then these must be external pollutants. For decades, it was believed that the interstellar medium, the tenuous gas between the stars, was the source of metals in these polluted white dwarfs.
Farihi and his team used data from the Sloan Digital Sky Survey (SDSS), a project that aims to survey the sky in infrared light, imaging more than 100 million objects and following up 1 million of these by obtaining their spectrum (dispersing the light by colour).
By examining the positions, motions and spectra of the white dwarfs identified in the SDSS, Farihi and his team show that this is no longer a viable theory. Instead, rocky planetary debris is almost certainly the culprit in most or all cases.
The new work indicates that at least 3% and perhaps as much as 20% of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planets with a total mass of about that of a 140 km diameter asteroid.
This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build terrestrial planetary systems.  Astronomers are thus playing the role of celestial archaeologists by studying the 'ruins' of rocky planets and or their building blocks.

Source RAS