Once, most of exoplanets that astronomers spotted were giants, but now they’re seeing more and more new planets with masses not far off from the Earth’s. One of those newly found Earth-like exoplanets, however, may not have always been so similar to our own world: An astronomer made the case last week that the small, sweltering planet was once a mighty gas giant that shrank.

Astronomers discovered Corot-7b in September. Its diameter is roughly 1.7 times that of Earth. Based on its size and mass, its density is similar to Earth’s, indicating that it is a rocky Earth-like orb [ABC News]. But the comparisons end there. While it’s rocky like Earth, this fiery hellhole is no place for life. It orbits its star at a distance of only 1.6 million miles (we’re presently at a much more comfortable 93 million miles from our sun) and completes a revolution in only 20 hours’ time. And, NASA’s Brian Jackson argued at last week’s American Astronomical Society meeting, Corot-7b is probably just a shell of its former self, and once was a type of gas giant called a “hot Jupiter.”

Given the planet’s proximity to its star, Jackson says it would be subject to a constant blast of heat that robs it of its mass. Rock vaporized by the extreme temperatures could escape the atmosphere of Corot-7b, and the planet would’ve steadily lost mass as it moved closer to its star. It could be shedding half an Earth mass every billion years. Extrapolating backward in time, Jackson concludes that the planet may have started as a gas-giant world more akin to Jupiter or Saturn, and that its light elements were driven off [Sky & Telescope]. The gas giant case isn’t clinched; one could also argue that the planet was always rock, and just slowly lost mass over the years. Either way “this planet is disappearing before our eyes,” Dr. Jackson said in a statement [ABC News].

Elsewhere at the AAS meeting, astronomers announced a new find: the second-smallest exoplanet ever, given a mouthful of a moniker in HD156668b. The team of astronomers who discovered HD156668b used one of two Keck telescopes at the 4,145-meter (13,600-foot) summit of Mount Mauna Kea in Hawaii. The astronomers used the so-called wobble method, which measures the gravitational effects of a planet on its star [AFP]. This new world is some 80 light-years away in the Hercules constellation. It’s only four times more massive than Earth.

With the exoplanet tally now well past 400, and the planet-hunting Kepler telescope starting to spot its first distant orbs, expect the announcements to keep coming. Maybe soon we’ll even find an Earth-like world that isn’t unbelievably sizzling hot.

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Image: ESO. Artist’s impression of COROT-7b.