Discovered: The Most Adorable Planet Yet

A newly discovered world orbiting a distant star makes us consider anew how we define a planet at all

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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.

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what a name for an ugly planet and I don't think its real any ways


Adorable? They don't know what it looks like - and those with extremely close orbits to their suns would tend to be big rocks with no atmosphere - like the Moon or Mercury.  So even Kepler-37d, shown in the picture as having an Earth- or Venus-like atmosphere is almost surely Moon-like due to it's 40 day "year".

mrbomb13 1 Like

...and how exactly is that planet "adorable" again?  

Unless Michael Lemonick is a fairy, that title is akin to a girl talking about her 'cute, adorable puppy.'

Nice going, Tinkerbell.


Astronomers need change the criteria for calling something a planet. If we found life on Pluto, would it still not be a planet regardless of its neighborhood? Actually there is indeed life on Pluto - see ufocoverup,org for more info


@DavidG There is no reason that life has to be confined to planets. If we ever find other life in our own solar system, chances are high it'll be on Titan, Europa or Enceladus. 


Lemonick is correct, the reason Pluto doesn't fit criterion 3 is where it is, not what it is. One absurdity of the IAU's logic is that if you put identical objects in different orbits-- say identical Earth's in the orbits of the 9 classical planets, only the first 7 would qualify--because the equations that criterion 3 are based on bias against planets far from their stars. This kind of craziness is why so many planetary scientist, like me, mock the IAU definition and just ignore it. In fact, planetary scientists now regularly call the Moon, and the large planet-scale moons of Jupiter, Saturn, and Neptune planets. 37B qualifies too. You see, to many planetary scientists, deciding if the object is a planet or not isn't about where the object is, it's about what the object is.

teviet 1 Like

@AlanSternThe logic is not absurd, it just recognizes that the term "planet" has always referred to both intrinsic properties (what it is) and extrinsic properties (where it is).  You are trying to conflate the more specific term "planet" with the more general category "planetary-mass object" (PMO).  Our Solar system contains some 32 known PMOs of which 8 are planets, 5 are dwarf planets, and 19 are major satellites.  It's a hierarchical classification.

So if we accept your definition of "planet" as equivalent to PMO, what would you propose as the correct term for a "planet that is not a satellite or member of a belt population"?


If you put the Earth too close to the Sun it would lose mass too. SO would Jupiter. Tyson's Pluto argument is entirely fallacious.


@AlanStern Granted that planets tend to get their atmospheres blown off by the solar winds at close range, but why would Earth lose mass now, if it spiraled inward?

There's a lot of "hot Jupiters" out there suggesting it's possible for pretty massive planets to orbit shockingly close to their stars.

Also, I thought consensus now is that Mercury's small size is due to having most of its crust blown off by an ancient impact, like the one that skimmed off our Moon and the one that took a bite out of Mars' North Pole?


The final statement of the article, So the real reason Pluto isn’t a planet, evidently, is that it lives in the wrong part of town, is inaccurate. Neil deGrasse Tyson, the man most fundamentally responsible for Pluto's demotion explained, that due to Pluto's high ice content if it were any closer to the sun, some of the ice would likely melt, making Pluto even small, and increasing the likelihood it would be unable to "clear it's neighborhood."