Scientists Find Smallest Exoplanets Yet

BY Jenny Marder  December 20, 2011 at 3:19 PM EDT

This ‘planet line-up’ displays the first two Earth-size extrasolar planets, Kepler-20 e and Kepler-20 f, together with the Earth and Venus, ranked by their size. Image by Tim Pyle.

Scientists have found two fiery hot, Earth-sized exoplanets whizzing around a sunlike star.

They are believed to be the smallest planets ever found beyond our solar system, and part of the multiplanet system, Kepler-20, located 1,000 light years away in the constellation, Lyra. Like all planets detected in this system, they are closer to their planet star than Mercury is to our sun. This means that one year on Kepler 20 e, the smallest planet, goes by in six days.

It also means that both planets are too hot to support life. Researchers estimate that the larger of the two is a scalding 800 degrees Fahrenheit and the smaller one is even hotter – about 1,400 degrees Fahrenheit.

But the fact that scientists were able to detect Earth-sized planets is a milestone, and it brings planet watchers a step closer to the holy grail discovery: a planet small enough and temperate enough to host life.

“It’s the first discovery of an Earth-sized and a smaller-than-Earth-sized planet,” said Leslie Rogers, one of the authors of the study, which was published on Tuesday in the journal, Nature. “It’s too hot for water, it’s not in the habitable zone, but as the Kepler space telescope stares at stars, it will hopefully start to see more planets that are this size.”

One of the planets has a radius that’s only three percent larger than that of the Earth, making it “effectively Earth-sized,” Rogers said. The other is believed to be slightly smaller than Venus.

The team used the Kepler telescope, which identifies planets by studying dips in brightness as planets pass in front stars. Data suggests that the planets could be rocky, made of iron and silicate. The outer planet “could have developed a thick water vapor atmosphere,” according to the study.

Determining the composition of an exoplanet requires knowing the planet’s density. Scientists calculate planet density by measuring the gravitational pull of the planet on its parent star. But in this case, the planets are so small, they lie below the detection threshold of instruments used.

“All we can do at this point is make educated guesses as to what the densities might be using models,” said Guillermo Torres, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics and a study co-author.

Without knowing the mass, the composition is ambigious, but scientists can speculate that the planets are likely to be rocky due to their small size, he said.

“But whether they have water on the surface or even an atmosphere is speculation. It’s unlikely that the inner one would have an atmosphere, because it’s so close to its parent star. Any atmosphere it might have had would have long evaporated. The other one is farther out, and it could still manage to keep hold of an atmosphere, but we don’t know.”

Earlier this month, scientists found a planet that does lie in the habitable zone. Kepler-22b, 600 light years away, averages about 72 degrees and orbits its own sun every 290 days. Yet while it exists in a zone that could potentially support life, it’s bigger than two Earths. This means that it’s likely more like a scaled-down version of Neptune – a gas planet – than a scaled-up version of rocky Earth, Rogers said.

Still, the two discoveries combined bring planet scientists closer to what they’re looking for, Torres said.

“We know that we can find planets in the habitable zone, but that particular one was too large,” he said. “The two planets we’ve announced today are actually Earth-sized. However, they’re not in the habitable zone. So we’ve got one part but not the other. Hopefully the next big announcement will be a true Earth-sized planet, but in the habitable zone of its star.”