



Video: Watch the mysterious transit in action

The heavens have aligned in a way never seen before, with two exoplanets overlapping as they cross their star. The phenomenon is so new it doesn’t yet have a name.

Teruyuki Hirano of the University of Tokyo, Japan, and colleagues used data from the Kepler space telescope to probe KOI-94, a star seemingly orbited by four planets. A planet passing in front of a star, or transiting, causes the star’s light to momentarily dim: that’s how Kepler spots exoplanets. Two planets transiting at the same time dim the star even more, but if they also overlap there is a momentary increase in brightness as the planets cover less of the star.

This latter light pattern is exactly what Hirano’s team saw. It seems that one planet candidate, KOI-94.03, passed in front of the star and then the innermost candidate, KOI-94.01, passed between the two.


As both candidates must still be confirmed, another explanation for the light show is that a single planet passed in front of a dark starspot. But there is no evidence for spots on the surface of the star.

Exosyzygy mouthful

The uncertainty hasn’t stopped speculation over what to call the event. Hirano favours “planet-planet eclipse”, but that implies the total covering of one body by another:

KOI-94.01 is thought to be larger than KOI-94.03, so parts of all three bodies were visible. Another option is “double transit”, but that can include cases where two planets don’t overlap.

For that reason, “overlapping double transit” is favoured by Darin Ragozzine of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, who in 2010 suggested looking for the light pattern that Hirano’s team has now seen. That’s a mouthful, but not as hard to say as his other suggestion, “exosyzygy”, a play on the general term for three celestial bodies in a row, “syzygy”.

Now that we have seen two planets lining up in front of their star, what are the chances of spotting three? “That is basically never going to happen,” says Ragozzine. While Kepler has seen a few solar systems with three planets transiting simultaneously, the odds of them all lining up are extremely low.

Reference: arxiv.org/abs/1209.4362