Twin Planets: The Same, But Very Different

A newly discovered pair of worlds can reveal a lot about the odds of finding another Earth

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Artist rendition of the Kepler space telescope.

It’s often said—we said it again ourselves just last week, in fact—that our own Solar System isn’t as typical as scientists once imagined. We’ve got four small, close-in, rocky planets (Mercury through Mars) and four big, gassy ones, starting with Jupiter and proceeding out to Neptune. But the very first exoplanets ever found, back in the mid-1990’s, didn’t fit the mold at all: they were big and gaseous like Jupiter and Saturn, but orbiting so close to their stars that they were hotter than Mercury. More recently, astronomers have been finding something else we don’t have: so-called super-Earths, bigger than Earth but smaller than Neptune—and planetary theorists are pondering how these worlds might have formed and what they’re made of.

The goal, of course, is to find a mirror Earth, a planet with a diameter and mass similar to our own, orbiting a sun like our own, in a thermally comfortable spot where water could exist in a liquid state. That hasn’t happened yet, but astronomers are edging closer and closer, and with each new world they discover, they’re developing a better understanding of how different species of planets form and, by implication, our odds of actually finding an Earth 2.0. A brand new exoplanet discovery represents one more step in that direction.

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Speaking at the American Astronomical Society’s winter meeting just outside of Washington, DC, Harvard astrophysicist David Kipping has confirmed the existence of two worlds, both of them about 1.6 times the diameter of Earth, orbiting a red dwarf star known as Kepler 301. The mass of one of the planets is almost exactly the same as Earth’s, while the other is about three times heavier. Those vital stats reveal a lot. Although the planets are the same size, says Kipling,“one is clearly rocky, while the other is gaseous.” The gaseous planet, he adds, “is almost like a miniature Neptune.”

Understanding how a pair of twins could be at once so similar and yet so different is one reason this discovery could prove so important. “There’s been a big effort recently to find the dividing line in size between rocky and gaseous planets,” says Kipping. “But it may be there is no dividing line.” Planets, like people, may simply exist on a continuum of sizes and make-ups—and a lot can influence where they fall. It’s possible, for example, that the more massive of the two new planets was once  much larger, but its atmosphere was stripped away by the star’s X-ray emissions. At this point, nobody can say for sure.

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The new study is important too for the methods the researchers used. The physical size of the planets was determined in a straightforward enough way: just is it has for more than 3,000 candidate worlds so far, the Kepler space probe measured the fractional dimming of starlight that occurred as each planet passed in front of (or transited) its parent star.

But Kipping and his colleagues figured out the planets’ masses, and thus their densities, with a technique nobody even imagined was possible just a few years ago. As the planets whip around Kepler 301—the heavier once every 14 days, the lighter once every 23—they tug on each other gravitationally, forcing the other to speed up or slow down a tiny bit and pass in front of the star a few seconds earlier or a few second later than it should.

These changes are known as transit-timing variations (or TTVs), and by measuring them precisely, astronomers can figure out the gravitational pull of the planets doing the tugging. When you know gravity, you can determine mass.  The technique was developed by Harvard astronomer Matt Holman, who first used it in 2010 (although a much cruder version was employed as long ago as 1846 to predict the existence and location of the planet Neptune, based on anomalies in the orbit of Uranus).

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The more conventional way of weighing a planet—by measuring the subtle changes in starlight as an orbiting world tugs its star back and forth—works only with very bright or very nearby planetary systems. Most of the candidate planets identified by Kepler are relatively distant. But the TTV technique can measure a planet’s mass hundreds of light-years away from Earth. “What we’re doing here,” says Kipping, “is showcasing the full power of the TTV method.”

They’re also showcasing yet again that a major assumption astronomers were making as recently as 1995—that other solar systems would more or less resemble ours—was completely misguided. “Nature,” says Kipping, “continues to surprise us.” By now, that should hardly be a surprise.

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