Super-Earths a juicy target for new space telescope

The discovery of three “super-Earth” planets orbiting a dwarf star roughly 97 light years away provides a juicy target for the James Webb Space Telescope to be launched later this year, say US astronomers. 

In a paper posted on the pre-print science platform arXiv, a team of scientists led by Joseph Rodriguez from the Harvard-Smithsonian Centre for Astrophysics in Massachusetts, US, say the discovery affords a rare opportunity to investigate the dividing line between smaller rocky planets and larger gaseous ones.

The planets, dubbed GJ 9827-b, -c, and –d, all orbit a K-type dwarf star, and do so rapidly, with orbits that range between 1.2 and 6.2 Earth-days. The frequency of their orbit means that the new space telescope – a joint venture between NASA and the European and Canadian space agencies – will be able to monitor them many times as they move in front of their host star, potentially revealing a wide array of valuable information.

Rodriguez and colleagues are particularly excited about the discovery because two of them fall within a size range that so far seems rare – or at least elusive.

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To date, more than 3000 exoplanets have been identified, with the Kepler mission adding at least another 4500 candidates to the list.

The California Kepler Survey, operated by NASA, has so far logged precise radii for 2000 identified planets and produced a surprising result. Almost all of them fall in a range that tops out at one-and-a-half times the radius of Earth, or starts at two. 

This has led to the observation that so far all exoplanets seem to be either super-Earths or mini-Neptunes.

The key difference, of course, is that those on the Earth-side of the divide are rocky, and those on the Neptune side are gaseous.

One theory for the puzzling lack of intermediates is that the rocky “sub-Neptune” planets recorded so far orbit comparatively close to their host stars. This may mean that solar radiation burns off the thick gaseous envelopes that cloak their more distant neighbours, leaving only small rocky cores.

GJ 9827-b, at 1.64 Earth radii, and GJ 9827-d, at 2.08, fall between the two divisions, potentially affording strong opportunities to study the transitional zone between rocky Earths and gassy Neptunes. GJ 9827-c has a radius of 1.29 Earth equivalents, and should therefore be simply rocky.

The short orbit periods of the three planets, the researchers note, will enable repeated observations over a limited timespan.

“The planets span the transition from rocky to gaseous planets, so the characteristics of their atmospheres and interior structures may illuminate how the structure and composition of small planets change with radius,” the scientists write.

For more information about the James Webb Space Telescope, check out the article ‘Watch This Space’ in the next print edition of Cosmos, available in stores or via cosmosmagazine.com in October.