Published online 5 January 2010 | Nature 463, 15 (2010) | doi:10.1038/463015a

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Kepler finds its first planets

Early data hint at discoveries to come in the hunt for Earth-like worlds.

WASHINGTON DC

Kepler pins down planet size by tuning in to the music of the spheres.NASA/JPL-Caltech

Stars hum and throb, and the vibrations of this cosmic music could aid the NASA satellite Kepler in its goal of finding an Earth-like extrasolar planet.

On 4 January at a meeting of the American Astronomical Society in Washington DC, the Kepler team announced that it had identified five new planets. These are the first to be found by the 1-metre telescope, which stares continuously at one swathe of sky and looks for the dimming as a planet crosses a star and blocks some of its light. Hundreds more planet candidates await confirmation as the telescope gathers more data. These include some that orbit stars bright enough for their characteristic 'asteroseismology' vibrations to be detected, says Ronald Gilliland, a Kepler team member at the Space Telescope Science Institute in Baltimore, Maryland.

A precise understanding of these vibrations could allow astronomers to separate Earth-sized planets into two groups: those that are rocky and those that are watery, says Dimitar Sasselov, a co-investigator on the Kepler science team and an astronomer at Harvard University in Cambridge, Massachusetts. "It makes all the difference."

Because the core of an older star vibrates differently from that of a younger one, asteroseismology measurements can allow a precise determination of the age of the star system (and thus the planet). The data can also lead to a better estimate of the star's size — which in turn leads to more precision in the planet size. Gilliland says the extra precision could, when combined with ground-based measurements, help to determine the density of exoplanets as much as 50% better than before. Sasselov says that will be just enough of an improvement to discern the difference between a rocky planet like Earth, which is 0.06% water, and a water world like the recently discovered GJ 1214b, which is probably at least 50% water (D. Charbonneau et al. Nature 462, 891–894; 2009).

It is still early days for Kepler, which launched on 6 March 2009 from Cape Canaveral, Florida. The first five planets were discovered with just the first six weeks' science data, and they cross their parent stars repeatedly in short period orbits of a few days. Four are bigger than Jupiter — the largest planet in our Solar System — and one is about the size of Neptune.

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Orbs more like Earth will be seen as the team shifts its attention to smaller planets in longer period orbits.

But Kepler only measures size. To understand density the team needs to measure mass as well, which comes from follow-up observations by ground-based astronomers. Sasselov says that even the giant 10-metre Keck telescopes in Hawaii lack an instrument sensitive enough to confirm an Earth-like planet if Kepler saw it. He is building a new instrument that he hopes to have installed on the 4.2-metre William Herschel Telescope in the Canary Islands by 2011 or 2012 — about the time when Kepler should have Earth-analogue candidates to check. 

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