EXOPLANET EXPLORERS DISCOVERIES – A SIXTH PLANET IN THE K2-138 SYSTEM

Announced the discovery of K2-138. This was a system of five small planets around a K star (an orange dwarf star). The planets all have very short orbital periods (from 2.5 to 12.8 days! Recall that in our solar system the shortest period planet is Mercury, with a period of ~88 days) that form an unbroken chain of near-resonances. These resonances offer tantalizing clues as to how this system formed, a question we are still trying to answer for exoplanet systems in general. The resonances also beg the question – how far could the chain continue? This was the longest unbroken chain of near first-order resonances which had been found (by anyone, let alone citizen scientists!). At the time, we had hints of a sixth planet in the system. In the original data analysed by citizen scientists, there were two anomalous events that could not be accounted for by the five known planets – events that must have been caused by at least one, if not more, additional planets. If they were both due to a single additional planet, then we could predict when the next event caused by that planet would happen – and we did. We were awarded time on the NASA Spitzer Space Telescope at the predicted time, and BOOM. There it was. A third event, shown below, confirming that the two previous events were indeed caused by the same planet, a planet for which we now knew the size and period.

So, without further ado, I’d like to introduce K2-138 g! It is a planet just a little bit smaller than Neptune (which means it is slightly larger than the other five planets in the system, which are all between the size of Earth and Neptune). It has a period of about 42 days, which means it’s pretty warm (400 degrees K) and therefore not habitable. Also, very interestingly, it is not on the resonant chain – it’s significantly further out than the next planet in the chain would be. In fact, it’s far enough out that there is a noticeable gap – a gap that is big enough to hide more planets on the chain. If these planets exist, they don’t seem to be transiting, but that doesn’t mean they couldn’t be detected in other ways, including by measuring the effect of their presence on the other planets that do transit. The planet is being published in a forthcoming paper that will be led by Dr Kevin Hardegree-Ullman, a postdoctoral research fellow at Caltech/IPAC.

In the meantime, astronomers are still studying the previously identified planets, in particular to try to measure their masses. Having tightly packed systems that are near resonance like K2-138 provides a fantastic test-bed for examining all sorts of planet formation and migration theories, so we are excited to see what will come from this amazing system discovered by citizen scientists on Zooniverse in years to come!