by Seth Shostak, Senior Astronomer, SETI Institute
“The latest planets turned up by NASA’s Kepler telescope are- like the kids in Lake Wobegon- gratifyingly above average. These new worlds offer both promise and insights, because they’ve got traits that are both appealing and mildly disconcerting.
In the four years since its launch, Kepler has chalked up 122 new and confirmed planets. It’s also caught the scent of nearly three thousand additional objects, of which probably 80 percent or more will turn out to be other-worldly orbs. Compare this track record to the approximately 700 planets painstakingly rooted out by ground-based telescopes in the last 18 years, and you can appreciate why some astronomers refer to the space-based instrument as a planet factory- churning out new worlds faster than a Hong Kong tailor turns out suits.
But here’s the thing: Kepler can find small planets (even smaller than Mercury). And diminutive worlds are more likely to be rocky, and lapped by oceans and atmospheres. In the vernacular of “Star Trek,” these would be M-class planets: life-friendly oases where biology could begin and bumpy-faced Klingons might exist.
Three of the new Kepler worlds have both the right size and the right orbital distances to boast temperatures at which water would remain liquid, a circumstance often assumed to be life’s sine qua non. One of these planets orbits the star Kepler 69- which is comparable in brightness and size to our Sun. This possibly habitable planet is ingeniously named Kepler 69c. The other two worlds are the spawn of a dimmer star called Kepler 62. Its brood includes at least five planets, but the habitable ones are labeled Kepler 62e and Kepler 62f.
All three of these potentially habitable worlds are “Super Earths.” The term isn’t intended to suggest planets with azure skies, unpolluted oceans, and sympathetic inhabitants. Rather, it’s a reference to size. Super Earths have super girths, between 1 and roughly 2-1/2 times that of our own planet. Habitable, in principle- just a bit bulked up.
According to SETI Institute scientist Jon Jenkins, Super Earths are turning up more and more often. They dominate the new worlds now being found by Kepler. Now that’s a bit of a head scratcher, because in our own solar system the number of Super Earths is zero. There’s nothing between the size of Terra Firma and Neptune, which is 4 times larger than Earth.
So is our solar system just unlucky, like a family with eight kids but no girls? Or is there some deeper explanation for the absence of a Super Earth nearby? We don’t know. And this is an unexpected puzzle for those who wish to know what constitutes an “average” solar system.
The discovery of these three planets has also encouraged scientists who look for life in deep space. The number of potentially habitable worlds discovered beyond our solar system is currently 9, out of a total of 872 confirmed exoplanets. The math is dead simple: it seems that the frequency of planets able to support life is roughly one percent. In other words, a billion or more such worlds exist in our galaxy alone. That’s a lot of acreage, and it takes industrial-strength credulity to believe it’s all bleakly barren.
So will SETI experimenters fix their antennas on these new planets? Well, the answer’s as obvious as a lounge lizard: of course they will. But give consideration to the fact that alien astronomers could have scrutinized Earth for more than 4 billion years without detecting any radio signals, despite the fact that our world is the poster child for habitability. Lots of planetary systems will require examination before we can reasonably hope to find an alien transmission. Still, at least we know that suitable planets are not dauntingly rare.
And there’s something else that encourages me in the search for signals from these newly found members of the planetary bestiary. Kepler 62e has an orbital period of 122 days; Kepler 62f’s period is 267 days. Consequently, every 89 years these two seductive orbs line up with Earth. They’re connected to us in a straight line. If some sophisticated society has colonized both planets, then their back-and-forth communication signals- if any- will be aimed our way during this special moment. So in this case, the new discoveries clue us not only where we should hunt for signals, but when. And that might nicely improve the odds of finding Klingons.”
“I can never look now at the Milky Way without wondering from which of those banked clouds of stars the emissaries are coming. If you will pardon so commonplace a simile, we have set off the fire alarm and have nothing to do but to wait. I do not think we will have to wait for long.”
- Arthur C. Clarke, “The Sentinel”