NASA’s Kepler spacecraft, which launched on a planet-hunting mission in 2009, searched one small patch of the sky and identified more than 4,000 candidate exoplanets. These distant worlds orbit stars other than our own sun. Kepler’s survey was the first to provide a definitive look at the relative frequency of planets as a function of size. Its results suggested that small planets are much more common than big ones. Interestingly, the most common planets are those that are just a bit larger than Earth but smaller than Neptune — the so-called super-Earths.
There are no super-Earths in our own solar system. While astronomers nowadays are able to look across distant space, and learn something about the sizes and orbits of the super-Earths, they’d like to know … what are super-Earth made of?
A super-Earth could be a bigger version of our own Earth — mostly rocky, with an atmosphere. Or it could be a mini-Neptune, with a large rock-ice core encapsulated in a thick envelope of hydrogen and helium. Or a super-Earth might be a water world — a rocky core enveloped in a blanket of water and perhaps an atmosphere composed of steam (depending on the temperature of the planet).
Heather Knutson is an assistant professor of planetary science at Caltech. She and her students use space-based observatories like the Hubble and Spitzer Space Telescopes to learn more about super-Earths. Knutson said:
It’s really interesting to think about these planets because they could have so many different compositions, and knowing their composition will tell us a lot about how planets form.
For example, because planets in this size range acquire most of their mass by pulling in and incorporating solid material, water worlds initially must have formed far away from their parent stars, where temperatures were cold enough for water to freeze. Most of the super-Earths known today orbit very close to their host stars. If water-dominated super-Earths turn out to be common, it would indicate that most of these worlds did not form in their present locations but instead migrated in from more distant orbits.
Knutson and her team use orbiting telescopes to analyze the starlight that filters through a exoplanet atmospheres as these planets pass in front of their stars, as viewed from Earth. In this way, they’ve been able to characterize nearly two dozen of the gas giant exoplanets known as hot-Jupiters, showing that these sorts of worlds have water, carbon monoxide, hydrogen, helium—and potentially carbon dioxide and methane—in their atmospheres.
But what about the super-Earths? So far, only a few are close enough and orbiting bright enough stars for astronomers to study them with currently available telescopes and techniques.
The first super-Earth that the astronomical community targeted for atmospheric studies was GJ 1214b, in the constellation Ophiuchus. Based on its average density (determined from its mass and radius), it was clear from the start that the planet was not entirely rocky. However, its density could be equally well matched by either a primarily water composition or a Neptune-like composition with a rocky core surrounded by a thick gas envelope.
Information about the atmosphere could help astronomers determine which one it was: a mini-Neptune’s atmosphere should contain lots of molecular hydrogen, while a water world’s atmosphere should be water dominated.
GJ 1214b has been a popular target for the Hubble Space Telescope since its discovery in 2009. Disappointingly, after a first Hubble campaign led by researchers at the Harvard-Smithsonian Center for Astrophysics, the spectrum came back featureless—there were no chemical signatures in the atmosphere. After a second set of more sensitive observations led by researchers at the University of Chicago returned the same result, it became clear that a high cloud deck must be masking the signature of absorption from the planet’s atmosphere. Knutson said:
It’s exciting to know that there are clouds on the planet, but the clouds are getting in the way of what we actually wanted to know, which is what is this super-Earth made of?
Now Knutson’s team has studied a second super-Earth: HD 97658b, in the direction of the constellation Leo. They report their findings in the current issue of The Astrophysical Journal. The researchers used Hubble to measure the decrease in light when the planet passed in front of its parent star over a range of infrared wavelengths in order to detect small changes caused by water vapor in the planet’s atmosphere.
However, again the data came back featureless. One explanation is that HD 97658b is also enveloped in clouds. However, Knutson says, it is also possible that the planet has an atmosphere that is lacking hydrogen. Because such an atmosphere could be very compact, it would make the telltale fingerprints of water vapor and other molecules very small and hard to detect. She said:
Our data are not precise enough to tell whether it’s clouds or the absence of hydrogen in the atmosphere that’s causing the spectrum to be flat. This was just a quick first look to give us a rough idea of what the atmosphere looked like. Over the next year, we will use Hubble to observe this planet again in more detail. We hope those observations will provide a clear answer to the current mystery.
In the future, new surveys, such as NASA’s extended Kepler K2 mission and the Transiting Exoplanet Survey Satellite (TESS), slated for launch in 2017, should identify a large sample of new super-Earth targets.
Of course, she says, astronomers would love to study exoplanets the size of Earth, but these worlds are just a bit too small and too difficult to observe with Hubble and Spitzer. NASA’s James Webb Space Telescope, which is scheduled for launch in 2018, will provide the first opportunity to study more Earth-like worlds. She commented:
Super-Earths are at the edge of what we can study right now. But super-Earths are a good consolation prize—they’re interesting in their own right, and they give us a chance to explore new kinds of worlds with no analog in our own solar system.