Eight years ago, astronomers announced the discovery of 51 Pegasi b (51 Peg b), the first planet found circling a sun-like star beyond our solar system.
The planet is about the size of Jupiter, but it orbits far closer to its parent star than Mercury orbits the sun. A year on 51 Peg b lasts just four Earth days.
That got Brian Jackson, a researcher at the Carnegie Institution for Science in Washington DC, and colleagues curious if there were planets located even closer to their host stars. So they began sifting through nearly three years of data collected by NASA's Kepler space telescope.
The observatory, now sidelined by a positioning system failure, looked for slight dips in the amount of light coming from target stars, potential telltale signs of planets passing by, or transiting, relative to the telescope's line of sight.
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Kepler was designed to seek out Earth-like worlds fortuitously positioned from their parent stars for liquid surface water, the so-called "Goldlilocks" zone believed to be favorable for life.
But staring at 150,000 stars for four years turned up thousands of other potential planets, including four candidate planets with orbits 20 times closer than Mercury circles the sun.
Surface temperatures on these planets would soar past 3,000 degrees Fahrenheit -- hot enough to melt rock.
One suspected planet, referred to as KOI-1843, circles its host star in 4.2 hours -- an orbit that brings it 40 times closer to its star than Mercury orbits the sun -- with a surface temperatures of about 4,200 degrees Fahrenheit.
Conditions on these planets would be extreme. Surfaces on their day-sides are likely molten rock, setting up the bizarre prospect of rocky vapor atmospheres that transform into "rock snow" falling on their relatively cooler night-sides.
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"That could be very interesting," Jackson told reporters during a webcast press conference at the American Astronomical Society meeting in Denver this week.
"These molten rock lakes could shed a rocky vapor atmosphere that could go screaming around to the night side of the planet and then be deposited as sort of a rock snow as the rock vapor cools," Jackson said.
While life on these worlds is highly doubtful, finding extrasolar planets that skim their parent stars is helping scientists figure out how planets form and evolve.
"They didn't form there, almost definitely," Jackson told Discovery News.
For starters, temperatures in the original gas disks from which the planets formed would have been too high at the planets' current locations for them to form, Jackson pointed out.
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Also, the parent stars were much bigger in their youth, putting the planets inside the stars, he added.
"They must have formed much farther out and then were brought in by some processes or combination of processes. We don't really know how they got so close," Jackson said.
The research has been submitted to the Astrophysical Journal and appears in the online archive arxiv.org.