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In search for life, more planet 'candidates' are found. Are any just right?

For a planet to support life, it faces long odds: It has to be the right size, right composition, and right distance from its star. On Monday astronomers announced a trove of new planet 'candidates.'

By Staff writer / September 12, 2011

This undated handout image provided by The Europeans Southern Observatory shows a close-up of the sky around the star HD 85512. The Keplar spacecraft is in search of planet 'candidates' where life might be possible.

Photo illustration/The Europeans Southern Observatory/AP

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If finding Earth-size planets orbiting other stars at distances hospitable for life is the holy grail for planet hunters, teams of astronomers are uncovering a lot of potential chalices.

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One team, associated with NASA's Kepler mission, announced Monday that they have identified 1,781 planet "candidates" as they peer at a patch of sky covering more than 156,000 stars, in the constellation Cygnus.

Of those 121 appear to be orbiting in the habitable zones of their host stars – a region of space around the star where a planet's temperature would allow liquid water to pool on the surface. Some of these habitable-zone objects appear to be so-called super Earths, which tip the scales at between roughly twice Earth's mass to 10 times the mass of the Earth.

IN PICTURES: Planets

The Kepler team calls the objects candidates because they still require confirmation from ground-based astronomers using other techniques for detecting them. The team announced the latest update to its planetary census at a conference in Moran, Wyo., near Grand Teton National Park.

Kepler, a spacecraft trailing behind Earth in the planet's orbit around the sun, stares at its target stars simultaneously. It uses the so-called transit method to find planets, looking for the telltale dimming as a planet passes in front of its host star. To confirm that the observed object is a planet, ground-based astronomers look at the impact of the planet's motion on the star's spectrum. Of the 1,781 candidates Kepler has detected, 27 have been confirmed as planets so far.

Although the detection method Kepler uses allows scientist to make some broad inferences about a planet's size, temperature, or whether the planet is rocky or a ball of gas, the planets are too far away to make direct measurements of any atmosphere they might have.

The stars Kepler looks at are roughly 600 to 3,000 light-years away.

That is why other teams are looking much closer to home.

A group using a planet-hunting spectrometer bolted to the back of a telescope at the European Southern Observatory's La Silla Observatory in Chile reported at the same conference finding a super Earth orbiting just inside the inner edge of the habitable zone of a star labeled HD 85512. It is in the constellation Vela some 6.2 light-years from the sun.

The planet, with 3.6 times Earth's mass, orbits the star once every 58 days. By comparison, Mercury orbits the sun once every 88 days.

The team detected the planet with a technique that uses the wobble in a star's spectrum to track the planet's gravitational tug on its sun. The discovery was part of a project to see if upgrades to the spectrometer they are using, dubbed HARP, was up to the task of detecting planets with masses comparable to Earth's in the habitable zones of near-by stars.

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