Last time, ’s Kepler planet-finding mission found a planet in the right orbital location to support life, but it was the wrong size. This time, Kepler has found two planets of the right size but in the wrong location.
The Kepler team reported Dec. 20 that it has located an Earth-like star called Kepler-20 orbited by five planets, including two — Kepler-20E and -20F — that are the closest in size to Earth of any planets yet discovered outside our own Solar System. But they are so close to their own star that they are too hot to support life.
The other three are giant gas planets like Jupiter that lack the requisite hard surfaces or atmosphere likely to support life.
Both -20E and -20F are closer to Earth in size than any other planets yet discovered, including Kepler-22B, which was announced Dec. 5, says Francois Fressin of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and lead author of scientific papers on the newest findings. The Kepler-20 star system is in the constellation Lyra, 1,000 light years from our Sun.
Kepler-20E is a rocky orb of silicate and iron slightly smaller than Venus and 0.87 times the radius of Earth. But its 6.1-day orbit around its parent star makes its surface glow at 1,400F, hot enough to melt glass.
Its companion, Kepler-20F, is just 3% larger than Earth and also too close for comfort, orbiting every 19.6 days. Its estimated surface temperature of 800F is much like the planet Mercury’s.
Not much is known about Kepler-22B, which orbits in the so-called Goldilocks zone that is “not too hot, not too cold” to support life, but its size is probably all wrong to fit that bill, says Linda Elkins-Tanton, director of the Carnegie Institution for Sciences’ Department of Terrestrial Magnetism in Washington, an outside observer of the latest Kepler findings.
At 2.4 times the size of Earth, 22-B is presumably a large gas envelope without a rocky surface that would make it habitable, she says. The newest findings are right from the surface composition standpoint but too close to their parent star.
While astronomers are still analyzing Kepler’s data and combining it with ground observations to better understand these newest planetary finds, their biggest mystery is about the nature of Kepler-20’s planetary system. It is so unusual that science team member and astronomy professor David Charbonneau of Harvard is going to have to revise his lecture notes.
Unlike planets in our own Solar System, which can be neatly separated by an “ice line” between the smaller, rocky planets that orbit closest to the Sun and larger gaseous planets farther out where surface temperatures are much colder, the Kepler-20 system mixes the two types up.
“The architecture of the [Kepler 20] system is crazy,” Charbonneau says. First there is a Neptune-like gas bag, followed by -20E, then another Neptune, followed by -20F and finally another Neptune. “It’s big, little, big, little, big ... that we know of,” he says. “All five orbit within a distance of Mercury to the Sun.” Mercury is closest to the Sun.
Not surprisingly, current understanding of solar system formation is based on our own Solar System, where gravity brought rocky material together to form hard planets closest to the star and the less dense, gaseous giant planets orbited farther away from their star. “So we really are challenging scientists to figure out how they are formed,” Elkins-Tanton says of the Kepler-20 system.