The Real Space Oddities: Super-Earths and Jumbo Neptunes

Stuck in a solar system in which planets come in only two sizes—big and small—we're finally learning about all the ones in between

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L. Calçada / ESO

This artist’s impression shows how the newly discovered super-Earth surrounding the nearby star GJ1214 may look.

Science loves big sample groups. When you’re trying to run a good experiment, your results will be a lot more reliable if you have a couple thousand subjects instead of, say, a couple dozen. That’s one of the things that’s made understanding planets so difficult. There are only eight in our solar system, and they come in only two varieties: small, dense, rocky worlds—Earth, Venus, Mars and Mercury—and the much larger Jupiter, Saturn, Uranus and Neptune, made largely of gases surrounding a rocky core.

Now, with the Kepler Space Telescope and other orbital and ground-based observatories having put more than 1,000 exoplanets—or planets orbiting other stars—on the books, with thousands more flagged for almost certain confirmation, astronomers are at last becoming able to fill in the gaps between the two familiar species of worlds. A pair of papers just published in Nature, is the latest illustration of how far that science has come.

By making exquisitely careful measurements with the Hubble Space Telescope, one team of observers has looked closely at two planets that fall into size ranges that are not quite like any worlds in our solar system. The first named GJ 1214b is about 2.7 times the size of our home planet. That puts makes it a so-called super-Earth, intermediate in size between Earth and Neptune. The other, known as GJ 436b, is slightly bigger than Neptune. What makes them both particularly interesting is that GJ 1214b is almost certainly shrouded with high-altitude clouds, and GJ 436b, may be as well.

When GJ 1214b was first discovered in 2009, just 42 light-years from Earth, astronomers couldn’t decide exactly what it was—either a mini-Neptune, with an extended, gassy atmosphere, or a planet made largely of water, with a more compact atmosphere rich in water vapor. Astronomers tried to figure out which it was by watching as the planet passed in front of its star. Water vapor is opaque to certain colors of light (water vapor is a greenhouse gas on Earth, for example, because it blocks infrared light from escaping into space). So a water-vapor-rich atmosphere should make the planet seem physically bigger through some colored filters, as the vapor blocks starlight.

That’s not what they saw. There was absolutely no apparent size change no matter which filters the astronomers used, but that could have meant one of two very different things. Either the planet indeed appeared slightly larger through different filters, but the change was too small to be picked up by the observations, or the planet’s atmosphere was shrouded in high-altitude clouds, which prevented the astronomers from seeing the water-vapor effect at all. “Astronomers threw the kitchen sink at this planet for three years after its discovery,” says Laura Kreidberg, of the University of Chicago, lead author on the GJ 1214b paper, “but the nature of the atmosphere has remained an enigma.”

No more. The new Hubble observations were so precise that they could have seen even a tiny change in the planet’s apparent size. But even with so exceedingly sharp an eye using multiple filters trained on GJ 1214b, the planet remained just a single apparent size. That means that GJ 1214b is definitely shrouded with clouds—made not of water vapor, but rather of potassium chloride, or perhaps zinc sulfide.

The good news that the riddle has been at least partly solved, however, is also bad news, since clouds on another planet do the same thing they do on Earth—which is reduce visibility, and that can be a problem. In particular, astronomers believe the first hints of life on other planets will come from the detection of life’s byproducts—gases like oxygen, which couldn’t persist for any length of time without some sort of biological activity to replenish them periodically. But if high atmosphere clouds block the view of those lower atmosphere gasses, the planet’s true biological nature will remain hidden.

The worse news is that GJ 436b may also be cloud-shrouded. The fact that the planet is slightly bigger than Neptune, means it’s too large to be considered a super-Earth. At a mere 33 light-years away, however, it’s even closer than its smaller cousin, and even easier to observe.

Here, too, astronomers led by Caltech’s Heather Knutson watched for changes in the planet’s apparent size as they viewed it through different filters—and here, too, there was no evident difference. That could well point to a cloudy shroud as well, although the result isn’t quite as definitive here. “We’re really hoping [the clouds are] not everywhere,” says Knutson, “because clouds hide other things we want to know about.”

What is certain, is that what the researchers have learned studying these two planets opens the door to similar analyses of uncounted other worlds. “We’ve discovered a whole slew of exoplanets in this range,” says Julianne Moses, of the Space Science Institute in Boulder, Colorado, who wrote a commentary Nature about the discoveries, “but we have nothing like them in our solar system. Their densities are all over the place. Some clearly have solid surfaces, while others are clearly gaseous.”

If the clouds do part on planets like this, we already know that we’d be able to study their possible biology, thanks to an experiment conducted on the only world in the universe that we know harbors life: Earth. In 1993, Carl Sagan published his own paper in Nature reporting on an experiment he conducted with the Galileo spacecraft as it flew by Earth en route to Jupiter. When Galileo turned its instruments toward our atmosphere, Sagan reported, it detected oxygen and other so-called biosignatures. “These are strongly suggestive,” he wrote, “of life on Earth.” Unless the clouds intervene, there’s no reason astronomers won’t be able to do the same experiment one day with a planet circling a distant star.