【Economist】Exoplanets： Sorting the sky

That planets come in different species is clear. Why they do so is now emerging

THE starting-point of science is collecting: animals, plants, minerals, elements, even stars. Then, once a collection is large enough, patterns begin to emerge. Animals and plants fall into phylogenetic trees, minerals into crystal groups, elements into the periodic table, stars into the Hertzsprung-Russell diagram. Those patterns both require and suggest explanation. Thus, the theory of evolution, the science of crystallography, an understanding of the chemical bond and a description of how stars shine over their lifetimes have all emerged from the classification of collections. Now, it appears, something similar is happening to planets.

A quarter of a century ago only nine planets were known—those of the Solar System, a number subsequently reduced to eight by the demotion of Pluto. These nearby worlds have, however, now been joined by thousands more that orbit stars other than the sun. Many of those have been discovered or confirmed to exist by Kepler, an American space telescope launched in 2009 with the specific aim of finding small, potentially Earth-like bodies, as opposed to the plethora of big, heavy, Jupiter-like gas giants that formed the bulk of previous discoveries.

On June 19th Andrew Howard of the California Institute of Technology and his colleagues announced the latest batch of Kepler’s discoveries, 219 of them, including ten that are about the size of Earth and have similar surface temperatures, and might thus be capable of supporting life. They also announced the result of an analysis of all of Kepler’s haul, the thrust of which is that small planets seem to come in two distinct types. Which type a planet is depends on its exact size. But there is a marked discontinuity between the smaller and the larger type, which seems to reflect the way that mass and chemical composition interact in the swirling clouds of gas and dust that form planetary nurseries.

Mind the gap

One of Kepler’s early findings was that there is an abundance of objects intermediate in size between Earth, the fifth-largest planet in the Solar System, and Neptune, the fourth-largest (shown, to scale, above). Because Neptune’s diameter is four times Earth’s, however, that is a big gap to fill.

At the top end of the range are so-called mini-Neptunes. These are mostly gas, but are presumed to have cores made of rock and ice. At the bottom end are rocky objects with little or no atmosphere. These are the largest of the terrestrial planets, similar in composition to the inner planets of the Solar System, and are sometimes known as super-Earths. But if and how the two sorts overlap has never been clear. Part of the reason for this lack of clarity has been a lack of accurate measurements of exoplanetary diameters.

Kepler, which works by measuring the dip in a star’s light caused by a planet passing in front of it, cannot determine the size of that planet directly. Rather, it measures the relative sizes of planet and star. A star’s size is deduced from its spectrum. Hot stars, which shine white, are big and bright. Cool ones, which shine red, are small and dim. There are exceptions—old stars such as red giants and white dwarfs—but these are easily recognised.

That this relationship between luminosity (and therefore size) and temperature is fundamental to stellar nature was recognised just over a century ago by Ejnar Hertzsprung and Henry Russell. Their diagram, which plots it as a graph, is a good example of data collection producing patterns for theoreticians to work on. It means, for instance, that a precise spectrum will accurately indicate a star’s size and thus, if that star is orbited by a Kepler-detected planet, the planet’s diameter.

Until recently, most of the stars around which Kepler has made such discoveries had not had their spectra closely analysed. This has now changed thanks to the telescopes, among the largest in the world, of the Keck observatory in Hawaii. Using these, Dr Howard and his colleagues have measured the spectra, and thus the sizes, of 1,300 of these stars.

Adding the Keck and Kepler data together shows the distinction between mini-Neptunes and super-Earths quite clearly. The maximum diameter of rocky planets is 1.75 times that of Earth. The smallest mini-Neptunes are twice the diameter of Earth. The gap between the two (a 50% difference in volume) suggests bodies of intermediate size are unstable.

Turn down the volume

Dr Howard and his colleagues suspect the gap is caused by the way planetary atmospheres form. Their calculations suggest that the jump between a rocky planet with little or no atmosphere and a mini-Neptune with a large one requires the addition of only about 1% of the planet’s mass, in the form of hydrogen and helium. Since these are the two lightest elements, they provide lots of volume for little weight. And, since they are the most abundant elements, they are readily available.

Being light, however, means they are easily lost. This is crucial. Dr Howard and his team reckon the lack of objects in the gap between the biggest rocky planets and the smallest mini-Neptunes is a consequence of the bodies that would otherwise fill it having insufficient gravity to hold onto their atmospheres. Instead, radiation from their parent stars strips those atmospheres away.

The large number of mini-Neptunes around (almost every planetary system found by Kepler has at least one) does raise the question of why there are none in the Solar System. That will require more study, with better instruments. And the progress of just such an instrument, called PLATO, was announced by the European Space Agency on June 20th.

Lift-off is planned for 2026. PLATO will look for planets around hundreds of thousands of stars. Its main objective is to seek ones that might be supporting life. Like Kepler, though, it will add enormously to astronomers’ planet collection, and thus to the developing science of planetology.

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June 22nd 2017 | Science and technology | 992 words