Planets are like puppies. They’re born litters, they stay close to their mama (in this case, their star), and no two are exactly alike. There are big planets, medium planets, and then there are the true runts. It’s the runts that are often the cutest, and for those folks who look at planets that way, a truly adorable new world came along this week. It’s called Kepler-37b (O.K., so the name isn’t so cute), it orbits a star 200 light-years away, and not only does it give us a new appreciation for how diverse so-called exoplanets can be, it also causes us to address — again — just how we define a planet in the first place.
This flea of a world, whose discovery was announced in this week’s issue of Nature, was found, like nearly all of the 3,000-plus confirmed or suspected exoplanets, by the Kepler space telescope, which has been in orbit around the sun since 2009. Kepler does its work not by observing planets directly — they’re too far from Earth and too small for the telescope to see — but by detecting the slight dimming of starlight that takes place every time a planet transits, or passes in front of its star, blocking a bit of light.
Big planets block more light, little planets block less, and in this case the dimming is so subtle — a little over two-tenths of 1% — that the odds were against Kepler’s noticing at all. “It’s absolutely tiny,” says Thomas Barclay, of the NASA Ames Research Center, lead author of the discovery paper, struggling for the right superlative. Tiny is a relative term when you’re talking about planets, and the diameter of Kepler-37b is in fact about 2,400 miles (3,860 km). But that’s just two-thirds as big as the little planet Mercury and only about 10% bigger than the moon. So yep, tiny fits.
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Spotting 37b at all took a bit of luck. It’s not that the Kepler telescope lacks the proper sensitivity, but rather that stars flicker in brightness on their own. That includes our own sun, whose sunspots and flares keep its light output from being perfectly constant.
Kepler scientists knew this when they sent the probe into space, but they didn’t know — and couldn’t have — that our sun is quieter than most of its cousins. That made the job of finding small planets, including twins of Earth, harder than anyone expected, and it meant the mission had to be extended from its original 3.5 years in order to get a real handle on how many planets like ours really are out there.
But this particular star, similar to the sun, albeit about 25% smaller, is an exception: its light output is extraordinarily stable, making it easier to spot the telltale dimming caused by a passing planet. Even so, says Barclay, “it was quite astounding when we realized what we were seeing.”
Nobody should expect to find anything living on Kepler-37b. The planet orbits its star once every 11 days (Mercury, by contrast, which is scorchingly close to the fires of our sun, takes just under 88 days), so it’s both too small to have an atmosphere and too hot to sustain life even if it had one. The parent star is also orbited by at least two other planets: one, with a 21-day orbit, is about three-fourths the size of Earth, and the other is twice Earth’s size, orbiting once every 40 days.
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“We’re looking very carefully for other bodies that might be there,” says Barclay, but since Kepler mission rules call for at least three sightings before anyone can cry “planet,” worlds on longer orbits take longer to confirm. Nevertheless, he says, “If we’ve got three transiting planets already, there’s a high probability that we’ll see others if they exist.” That’s especially true for such a quiet star, which Barclay calls “an ideal test case.”
One crucial question remains, at least for those who worry about the technical definition of when an object can be called a true planet and when it’s relegated to the second-class status of “dwarf planet,” a fate that befell sad little Pluto back in 2006. As it happens, Barclay and his co-authors did think about the question. In fact, he says, “we talked about the issue in the first draft of our paper, but we were asked to remove that section before publication.”
The answer, he says, is that Kepler-37b might be small, but it fits the three criteria outlined by the International Astronomical Union for full planethood. First, it orbits its star (the moon Ganymede is bigger than Mercury, but since it orbits Jupiter and not the sun, it’s not a planet). Second, it’s roughly spherical. And third — the criterion that spelled doom for Pluto — simulations show that Kepler-37b has almost certainly “cleared its neighborhood,” meaning there are no other bodies of significant size that share its orbit around the star. Pluto’s orbit passes through the Kuiper belt, a ring of icy, rocky bodies that surrounds our solar system.
But there’s a catch that will undoubtedly make Pluto partisans see red. “The shorter the orbital period,” says Barclay, “the more chance an object has to clear its neighborhood. If Kepler-37b had an orbit like Pluto’s, it probably wouldn’t be considered a planet either.” So the real reason Pluto isn’t a planet, evidently, is that it lives in the wrong part of town.