IT IS the planetary system that gives two for the price of one. The star Kepler 62 hosts a pair of planets roughly the size of Earth. Both are orbiting in the star’s habitable zone, the region where temperatures should be neither too hot nor too cold, but just right for liquid water to exist (see diagram).
Known as Kepler 62e and Kepler 62f, these worlds are being hailed as the most life-friendly planets yet seen outside our solar system. But that doesn’t mean they would support life quite as we know it.
For one thing, the star is smaller and cooler than our sun, so at least one of the planets would need a potentially toxic atmosphere to keep warm. For another, some models suggest that the planets are covered in water, hinting that, if there are extraterrestrials, they might have evolved almost entirely in marine environments.
Advertisement
Confirming any of these ideas would require taking a more detailed look at the planets, but the system is 1200 light years away. That’s too far for us to take a peek at their atmospheres or get a definite picture of their composition with current technology. Still, the discovery brings us closer to seeing a true Earth twin, and it gives clues to what we may find in planetary systems closer to home as new telescopes come online.
“These worlds give clues to what we may find closer to home as new planet-hunters come online”
The two planets were discovered by a team working with NASA’s Kepler space telescope, which uses dips in starlight to deduce the presence of planets passing in front of stars. The team actually saw five planets – b, c, d, e and f – around Kepler 62. But only e and f are in the star’s habitable zone ().
These worlds are both about 1.5 times the size of Earth, which means they are probably rocky. Two Earth diameters is the cut-off below which planets should be solid, says of Harvard University. Larger than that and planets may be mini-Neptunes – small, gaseous worlds with no defined solid surfaces.
Of course, some astronomers think Kepler 62e and Kepler 62f don’t have solid surfaces. Theoretical models suggest both balls of rock are covered with liquid oceans (). “If you take Earth, keep the fraction of water the same and just make it bigger, you get enough water to cover the whole surface, because the volume is four times bigger but the surface doesn’t get four times bigger,” says Kaltenegger.
Kepler 62e is closer to the star than Kepler 62f, and the models hint that it may be hot and muggy all over, its sky filled with clouds. Kepler 62f, meanwhile, may need an atmosphere rich in powerful greenhouse gases, like carbon dioxide, to stay warm enough for liquid oceans.
If both planets support alien seas, they may be teeming with life, just as our oceans are, adds William Borucki, principal investigator for NASA’s Kepler mission and leader of the team that found Kepler 62’s planets. The worlds are also massive enough that they should hold on to thick atmospheres, so sea life could perhaps take to the air.
“We know that at least in our ocean, we have flying fish. They fly to get away from predators,” says Borucki. “So we might find that they’ve evolved birds on this ocean planet.”
Whether these marine life forms would develop intelligence and craft civilisations like ours is another question, though, because they would not have easy access to metals, electricity or fire.
Kaltenegger says it is also possible that the Kepler 62 planets have oceans and continents, like Earth. That raises some fascinating possibilities – including the idea that aliens from the two planets may already have communicated, or even met. “Imagine looking through a telescope to see another world with life just a few million miles from your own, or having the capability to travel between them on a regular basis,” says astronomer , also at Harvard. “I can’t think of a more powerful motivation to become a spacefaring society.”
“Imagine looking through a telescope to see another world with life a few million miles from you”
If both worlds host intelligent life, we might be able to snoop on a conversation between Kepler 62e and 62f, says Jon Jenkins, who is on the Kepler team and with the SETI Institute in Mountain View, California. If they are using radio signals about as powerful as communications broadcasts on Earth, telescopes here could pick them up. It might even be easier to detect a signal going between these planets than broadcasts from a single planet, adds SETI astronomer .
Colliding bullets
“If you wait until they’re lined up with respect to us, you might be able to listen in when we’re in the path of the beam,” Shostak says. “Some people say that hearing a signal from another world is like the odds of two bullets hitting each other. But that’s a lot easier to do if you know the bullets are being fired towards you.”
The Kepler team also announced last week another potentially life-friendly planet orbiting a star very similar to ours. Kepler 69c is about 1.7 times the size of Earth, so is also likely to be rocky (), says Kepler team member of the Bay Area Environmental Research Institute in Sonoma, California.
Kepler 69c orbits its star at about the same distance that Venus orbits the sun. The planet could also be a water world, or it could be a super-Venus, with a thick, toxic atmosphere that keeps the surface too hot for liquid oceans. Unfortunately, the Kepler 69 system is 2700 light-years away – again too distant for astronomers to determine the make-up of its atmosphere.
While they remain enigmatic, Kepler 62 and 69 are giving us a taste of what we might expect to find with future exoplanet surveys, such as NASA’s Transiting Exoplanet Survey Satellite (TESS), scheduled for launch in 2017. Kepler has been looking into deep space in one region of the sky, but TESS will use an array of wide-field cameras to scan roughly 2 million of the stars closest to ours, searching for Earth-size exoplanets in the habitable zones.
The new Kepler worlds also help to answer the question the mission set out to address: how many stars have potentially habitable planets?
“Finding these three planets – even if you ignore the others that Kepler has found – shows you it’s at least one in a thousand,” says Shostak. “That means there are about a billion habitable worlds in the Milky Way alone. Seems like a lot of real estate to me.”
This article appeared in print under the headline “Water worlds swell hopes of finding life”
