What’s with all the mini-Neptunes?
As astronomers peer deep into our galaxy, studying starlight for the telltale twinkle of orbiting planets, they are seeing something unexpected: Most planets detected so far appear to be smaller than Neptune but larger than Earth, in a size range that does not exist in our solar system.
Here we have four rocky planets, of which Earth is the largest, and two gas giants, Jupiter and Saturn. Uranus and Neptune, sometimes lumped with Jupiter and Saturn and sometimes labeled ice giants, have rocky cores with gaseous envelopes. The smaller of the two, Neptune, has a radius four times that of Earth. Pluto, recently demoted from planet status, is a dwarf planet.
Throw in assorted moons, asteroids, comets and Pluto-like objects, and that’s our idea of a solar system. But astronomers have found that most planets detected around distant stars are what you might call off-size worlds, as if rounded up from an outlet mall.
The findings were presented Monday at the annual meeting of the American Astronomical Society, held at National Harbor.
Astronomer Geoff Marcy of the University of California at Berkeley presented data showing that about 85 percent of planets found by NASA’s Kepler space telescope are “mini-Neptunes” or “super-Earths.” Marcy noted that these planets orbit close to their parent stars and that it is possible, with advances in instrument sensitivity, that scientists will discover an abundance of small, rocky planets at more distant orbits.
But that’s not what we see so far. Instead, there seems to be a distinct cosmic preference for this intermediate range of planet. These planets also seem to follow a pronounced pattern: Up to about twice the diameter of Earth, they are rocky and dense. But beyond that the average density plummets dramatically, suggesting that the bigger worlds are enveloped in gas.
This observation matches the theory of planet formation, in which there’s a limit to how large a purely rocky world can get. Shovel more and more dirt onto a big rocky planet and it doesn’t get any bigger but rather compresses because of gravity, Marcy said.
The Kepler telescope, launched in 2009, discovered the planets by observing the periodic dimming of starlight as planets transit across the disk of the star. Since then, scientists have been reexamining the parent stars with different methods. Marcy and his colleagues, for example, used radial velocity measurements, observing the Doppler shift in starlight as stars are tugged by the gravity of orbiting planets.
Scientists have been on a roll in the discovery of planets beyond our solar system — hundreds have been found, and billions inferred through statistical extrapolation. Increasingly, it is obvious that this is a universe with a generous amount of planetary real estate. The next great leap is characterizing these places to learn more about their atmospheres, surfaces, density and so on.
Could there be life on such planets? That remains a matter of speculation, hampered by a limited understanding of what life requires. The larger, gaseous mini-Neptunes would have, at their surface at the bottom of the atmosphere, extremely high atmospheric pressures, akin to what would be experienced at the bottom of an ultra-deep ocean. That said, life exists around deep-sea hydrothermal vents on Earth, so it is not an impossibility.
“We know very little about how life got started and in what environments it might flourish,” Marcy said. “We’re really in the dark about the biology aspect of these planets.”
Another astronomer, David Kipping of the Harvard-Smithsonian Center for Astrophysics, announced a newly discovered planet some 200 light-years away — fairly close in the galactic scheme of things — that is a bit of an anomaly. It has the same mass as Earth but is more than half again as large, suggesting that it is fluffy, with an extensive atmosphere. It’s “nothing like the Earth,” Kipping said.
A University of Chicago graduate student in astronomy, Laura Kreidberg, presented data about the atmosphere of “everybody’s favorite super-Earth,” officially named GJ 1214b, which shows signs of having a layer of clouds of unknown composition. Astronomical observations have ruled out the possibility that they are made of water, carbon dioxide or methane. “I put my money on a layer of haze high up in the atmosphere, made out of soot . . . kind of like smog, actually,” Kreidberg said.
One possible source of the cloud droplets is potassium chloride, which she said is pink when in liquid form. “It could be a pink planet,” she said.