Bring on the Exo-Earths!

Very soon the Kepler science team will announce finding an Earth-sized planet orbiting in the habitable zone of a normal star. (This article was just going to press when a team using ground-based scopes announced a super-Earth in the habitable zone of the star Gliese 581; see page 16.) There will be a press conference with a dramatic revelation and giant headlines: “Kepler Spacecraft Finds a Planet that Could Be Another Earth!” Perhaps the President will congratulate the team. Yet in a certain sense this announcement will be anticlimactic. No scientist, or astronomy fan who follows the news, will be surprised.

As of this writing, none of the nearly 500 known exoplanets are Earth-sized, but that is surely about to change. It’s easier to find giant worlds than small ones, and it’s easier to find close-in, hotter planets on tight, fast orbits around their stars. NASA’s Kepler spacecraft has been operating since early 2009, keenly recording the slight dimming of starlight from orbiting planets passing in front, but it takes multiple transits before astronomers can be certain they have found an Earth-sized world. 

Given this fact, and the significance of announcing “another Earth,” the Kepler team is understandably being cagey about what it has found. Nobody wants to make a historic announcement that turns out to be wrong, or a slip that ruins the press briefing. Yet the team has already announced 7 planets, and it would be beyond strange for our universe to make giants and super-Earths and not also a lot of smaller ones. Earth-sized worlds must be out there. 

The big mystery about Earth-size planets is no longer existence, but demographics: How many are there and in what kinds of orbits around what kinds of stars? These cumulative findings are more scientifically useful but not as prone to generate headlines. But when we start to learn some of the details of the population of other maybe-Earths, then astronomers can get to work on figuring out what conditions are really like on these worlds. 

Just because a planet is Earth-sized, does that mean it’s Earth-like? This might seem to be a silly semantic question, on a par with, “Is a dwarf planet a planet?” But it has generated some heated and interesting debate. What do we really mean by Earth-like? Perhaps a good criterion would be the continuous presence of stable surface water over billions of years. Even among that subset there will, undoubtedly, be a wide variety of worlds, and probably not many that we would actually confuse with Earth upon even casual inspection. But these worlds would be habitable for our kind of carbon-based, water-immersed life and so they seem to qualify as “Earth-like.”

We only have two examples of Earth-sized planets: Earth and Venus. Should we conclude from this sample that 50% of all Earth-sized planets are Earth-like? Of course not. We need more data. Bring on the other Earth-sized planets so we can start to model, predict, and find ways to observe their atmospheres and climates, to find out what kind of worlds they are. That’s when, for the question of comparative climatology, the real work, and the fun, will begin.

This article originally appeared in print in the December 2010 issue of Sky & Telescope. Subscribe to Sky & Telescope.