The important thing about yesterday’s announcement of seven planets around TRAPPIST-1 wasn’t just the number, although that many Earth-size worlds circling one star is, at least for now, a record. It’s the observability of this particular solar system that matters. Astronomers who study exoplanets are a little like the guy who loses his keys in the middle of the block, but looks under the corner streetlight because the light is better. For now, we’re limited to studying the planets that are best suited to our methods.
That’s where the star called TRAPPIST-1 (after the relatively small telescope in Chile that discovered planets crossing its surface) really shines. It’s fairly close by astronomical standards, just 39 light years away. The star is small and dim compared to our sun, which means that the darkening of its surface as planets cross in front of (transit) its disk is easier to spot. Lucky for astronomers, the seven planets are all orbiting very close to the star, and in the right orientation, so that they we can see lots of transits in a short amount of time. Even luckier, the planets are bunched close enough together that they exert a substantial gravitational tug on each other. That shows up as tiny variations in the timing of the transits as the planets repeatedly circle their star. Looking at all this information together, scientists can figure out the planets’ masses as well as their sizes, something they can’t always do. And that allows them to infer that these are rocky, Earth-size planets.
Does Earth-size mean Earth-like? Not necessarily, despite the headlines in many newspapers this morning. It’s true, a few of the TRAPPIST-1 planets are within their star’s “habitable zone,” where liquid water—and therefore, theoretically, life—could exist. But just being in that zone is no guarantee of habitability. Right now, we can only speculate about life in the TRAPPIST-1 system.
Still, because of the advantages listed above, it will be a great natural laboratory for what might be called “comparative exoplanetology”—particularly when advanced instruments like the James Webb Space Telescope come online that allow astronomers to study the planets’ atmospheres, if they have them.
It turns out, though, that we already have such a laboratory much closer to home. Earth, its moon, Venus, and Mars are all in, or nearly in, or used to be in, the sun’s “habitable zone.” Only Earth has life on its surface today. How come? Finding out what characteristics make a planet habitable, and then what makes a habitable planet inhabited, is a very tough research problem, and scientists will need all the data they can get.
That’s why NASA’s decision last month to fund yet another pair of asteroid missions, instead of either of two proposed missions to Venus, was so disappointing. Nothing against asteroids. It’s just that some planetary scientists are starting to wonder if the agency has gotten so risk-averse and budget-conscious that it always favors safe, incremental missions (asteroids are relatively cheap and easy to explore) over ones that push the envelope in the hope of a bigger scientific payoff (Venus probes are doable, but can be riskier technically). NASA hasn’t launched a dedicated Venus mission in almost 30 years, and it has never sent a lander. Only one country has, and that was so long ago that the nation that sent it—the Soviet Union—doesn’t even exist anymore.
If scientists were to find a planet exactly like Venus around a distant star, they’d be jumping up and down with excitement. It’s about the same size and mass as Earth. It has an atmosphere. It’s roughly the same distance from the sun as Earth. Habitable, right? No. Maybe in the past, but not today, due to its poisonous (to us) atmosphere and oven-like surface temperatures. What happened? We won’t know until we send more orbiters and landers. And there’s no shortage of ideas for such missions.
Understanding new solar systems like the one announced yesterday may require us to learn more about our own planetary neighborhood first. Exoplanet-ologists need planetologists, and vice versa. After all, it’s going to be a very, very long time before we can land a probe on TRAPPIST-1g. We could send one to Venus any time we choose.