Video: Exoplanets overlap as they cross their star
Planet-watchers, mark your calendars for April Fool鈥檚 Day, 2026. That鈥檚 when the next known case of a rare celestial alignment in an alien solar system is due. The event involves two planets overlapping as they cross their star 鈥� and has only ever been seen once before.
In 2012, Teruyuki Hirano of the Tokyo Institute of Technology, Japan, and colleagues reported the first sighting of this particular alignment. One name coined to describe it was the tongue-twisting exosyzygy: 鈥渟yzygy鈥� because that means three celestial bodies in a row and 鈥渆xo鈥� as it happens outside our solar system, as in exoplanet.
The team made the discovery by analysing data from NASA鈥檚 Kepler space telescope. Before it broke down last year, Kepler spotted exoplanets by watching for a telltale dimming in light as a planet transits, or passes in front of, its star.
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When two planets transit at the same time, they block even more of the star鈥檚 light. If they overlap for a time during the transit, however, there is a momentary increase in brightness because the aligned planets cover less of the star.
Celestial forecast
Hirano鈥檚 team looked at Kepler observations of KOI-94, a star with four unconfirmed planets, and found one example of this pattern, suggesting two planets had overlapped while transiting the star on 14 January 2010. Since then, the planets have been confirmed and the star has been renamed Kepler-89.
Eclipses of the sun and moon, along with transits of the sun by our planetary neighbours, are noteworthy in themselves, but this is the equivalent of seeing Mercury and Venus transit the sun at the same time 鈥� and overlap in the process.
Although there was a , no similar events have been seen around other stars. That suggests they are very rare, so Hirano鈥檚 team decided to predict when the next one might occur around Kepler-89.
They analysed the future orbits of the three largest planets 鈥� the smallest and nearest to the star had a negligible effect, the team decided 鈥� and discovered that the two outermost planets will transit and overlap again on 1 April 2026, for around two hours.
Hints from gravity
Gravitational interactions between the planets could throw a crossing off-course, but the researchers did take these into account and concluded they are very unlikely to do so. But Kepler-89 could have a fifth, as yet unseen planet, that would tug the other two apart. So if the event doesn鈥檛 happen, that would be a novel indication that a new world might be lurking around Kepler-89.
鈥淪ince Kepler was forced to end its original mission, it became difficult for us to refine the system parameters and to search for another unknown planet,鈥� says Hirano, who has already circled the date. 鈥淚 hope to observe but am not sure that I can get an observing time with an appropriate telescope for the whole event.鈥�
鈥淚 think they did a great job,鈥� says Darin Ragozzine of the Florida Institute of Technology in Melbourne, who first suggested looking for such events. 鈥淲e don鈥檛 learn anything from the predictions of the model. However, we do know when to point our best telescopes, and that鈥檚 valuable.鈥�
No agreed name
The event is so unusual that there is still disagreement about what to call it. Hirano鈥檚 team uses 鈥減lanet-planet eclipse鈥�, but that can be confusing because it suggests one planet entirely blocks the other, which isn鈥檛 the case. 鈥淚鈥檓 not sure if the term PPE is actually accepted in the community,鈥� Hirano says.
Ragozzine coined the term 鈥渆xosyzygy鈥�, but it has yet to catch on. 鈥淚t may be too late to start retroactively calling these exosyzygies, although it is an awesome name,鈥� he says.
Scrabble players might want to take note, given the profusion of high-scoring letters in the word.
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