Planetology comes of age

LORD RUTHERFORD, who discovered the atomic nucleus, once said that “all science is either physics or stamp collecting.” He had a point, but was being a bit unfair to stamp collecting. To come up with a theory that brings meaning to a pile of observations—whether of Galapagos finches, planetary orbits or pea plants—you have to do the hard graft of collecting those observations in the first place.

Take planetary science. For almost all its history, it could study only the eight planets that make up the local solar system. But the boom in exoplanet research over the past decade or so has furnished the field with a wealth of data from elsewhere in the galaxy. Much of this has come from a specially designed space telescope called Kepler, some of the discoveries of which are illustrated in the artist’s impression above, along with objects from the local solar system, for comparison. Kepler’s discoveries, and others, have done plenty of exciting violence to old theories of what planets are and how they form. Several papers discussing what is happening were presented at the meeting of the American Astronomical Society which took place this week in Washington, DC.

Astronomers are particularly interested in planets intermediate in size between rocky Earth and gassy Neptune, which along with Uranus is one of the solar system’s two “ice giants”, and which has a radius 3.8 times that of Earth and is around 17 times as massive. Planets of this intermediate size are common, but because the local solar system does not host one, they are also mysterious. Are they scaled-up Earths, scaled-down Neptunes or a mixture of the two? And, if they are a mixture, where is the boundary between the rocky ones, known as super-Earths, and the gaseous ones, known as mini-Neptunes?

Testing the boundaries

Several recent studies have tried to answer these questions. One, presented to the meeting by Yoram Lithwick, an astronomer at Northwestern University in Chicago, analysed 64 worlds. A second, described by Geoff Marcy, a veteran exoplanet hunter at the University of California, Berkeley, considered 42. The tentative consensus is that there is, indeed, a mixture—and that the boundary between the two lies at around twice the diameter of Earth. Planets smaller than that are likely to be rocky, with thin or non-existent atmospheres. Larger worlds are gassy, with their solid surfaces buried beneath deep blankets of hydrogen and helium.

But nature is full of surprises, and no sooner had this rule been suggested than it was flouted. Kepler 314c, described by David Kipping, an exoplanetologist at the Harvard-Smithsonian Centre for Astrophysics, has a mass somewhere between 0.7 and 1.4 times that of Earth, making it one of the lightest planets so far discovered. But its modest mass belies its impressive size: it has 1.6 times the diameter of Earth. This means it is only slightly denser than water, and that suggests it possesses the kind of deep, puffy atmosphere Drs Marcy and Lithwick would reserve for much larger planets.

One clue as to how this happened might lie in the fact that Kepler 314c orbits close to its parent star—its “year” is 23 terrestrial days long. This suggests to Dr Kipping that it formed as a standard mini-Neptune, then moved into a closer orbit and had much of its atmosphere boiled off by the radiation of its parent.

For now, that is just a hypothesis, inferred from a handful of measurements of mass and radius. But Dr Kipping says it might be possible to confirm it by taking pictures of the planet directly, using a big space telescope like the Hubble, or the soon-to-be-launched James Webb.

Such “direct imaging” is on the edge of what is presently possible, and a few especially suitable planets have already had their pictures snapped. But the conference also heard from those running the Gemini Planet Imager, a camera bolted to one of the twin Gemini telescopes, in Chile (the other is in Hawaii), explicitly to take pictures of exoplanets and the discs of gas and dust from which they condense. The instrument—which uses a computer-controlled deformable mirror to counteract the atmospheric disturbances that normally muddy pictures taken from the ground—saw its first light in November.

Anti-diluvian

Astrobiologists are particularly interested in super-Earths since, unlike mini-Neptunes, their thin atmospheres, and consequently low atmospheric pressures, mean they could be habitable—assuming, that is, that they have any dry land. For though life can certainly evolve in oceans, as it did on Earth, it probably needs the chemical and climate-regulating contributions of continents to do so.

Unfortunately, many exoplanetologists think super-Earths will be covered entirely by oceans, since their high gravity will make it hard for areas of light rocks (such as those which form continents on Earth) to rise above sea level. However, work described by Nicolas Cowan and Dorian Abbot, of Northwestern University and the University of Chicago respectively, suggests that such “waterworlds” may be less common than feared. They argued that, as on Earth, a great deal of water will be locked up within such planets’ mantles, forced there by the very gravity that others worry will drown their surfaces.

For now, like Dr Kipping’s theory of evaporating atmospheres, that remains conjecture. But a sufficiently powerful telescope could help check it by collecting enough light to build a crude map of an alien planet: not enough, perhaps, to see the outlines of continents, but sufficient to determine how much of its surface was solid and how much liquid. In 2009 this idea was tested by a group of scientists including Dr Cowan, who used a space probe called Deep Impact to take blurry pictures of Earth, of a sort that a future telescope might snap of alien worlds. From that, they were able to tell the dry bits of the planet from the wet ones.

When (or if) such a telescope might be built is, alas, a sore subject among planet-hunters. A decade ago NASA laid out plans to construct a colossal instrument called the Terrestrial Planet Finder. But the $5 billion project foundered amid budget cuts and protests from other parts of astronomy. A European equivalent, called Darwin, is similarly moribund. In fact, amid all the excitement and new discoveries, the rate of stamp collecting has suddenly slowed right down. In May 2013 Kepler stopped working properly. Soon after, another instrument, called CoRoT, failed as well. Fortunately, there are still a lot of data from these machines waiting to be analysed. But, as every philatelist knows, more stamps will always be welcome.