Seeking extraterrestrial eclipses

On 21 August sky watchers across the US will get to witness a pockmarked ball of rock pass directly in front of a star that’s both 400 times as large and 400 times as distant. For a lucky group of spectators, those celestial orbs will be in perfect alignment for 2 minutes and 40 seconds, inducing temporary nightfall and an ephemeral glimpse of towering jets of plasma erupting from the star’s surface.

When it comes to eclipse viewing, there’s no better place and time than Planet Earth right now. No other planet–moon combination in the solar system can deliver the crossing of two spheres of nearly identical apparent size. And within several hundred million years, the Moon will have scooted too far away from Earth to completely obscure the solar disk.

Although Earth features an unmatched eclipse experience, other stars, planets, and moons do offer some awe-inducing celestial alignments. In a paper in July’s Monthly Notices of the Royal Astronomical Society, astronomers Elmé Breedt and Dimitri Veras of the University of Warwick in England explore syzygies—alignments of three gravitationally bound objects—within the solar system, as well as in a few select extrasolar systems.

Unsurprisingly, Breedt (who recently moved to the University of Cambridge) and Veras find that Earthlings enjoy the most exciting eclipses in the solar system. Sure, visitors to Mars would witness daily eclipses (except at the poles) from the planet’s two low-lying moons, Phobos and Deimos. But the moons are puny, the events are fleeting, and the Sun looks smaller in the sky than it does from Earth. Jupiter’s four Galilean moons are large enough to totally block out the Sun from the perspective of a viewer flying through the clouds of the giant planet. But again, the Sun would appear too small in the sky to make the event much of a spectacle.

The fare gets far more exciting when you venture outside the solar system to some recently discovered exoplanet systems. In their paper Breedt and Veras describe the truly unusual experience that can be had in the Kepler-47 system : an eclipse brought on by the shadow of a star. The two planets there, which are likely too hefty to support life, orbit a binary system made up of a main sequence star and a red dwarf. Any inhabitants of those planets would occasionally see their primary star obscured by its cooler, and thus much darker, companion star.

The TRAPPIST-1 system , located 39 light-years away in the constellation Aquarius, has all the makings of a great eclipse experience. First, there’s a large swath of sun to be concealed. Although the radius of the red dwarf star TRAPPIST-1 is only a tenth that of the Sun, the seven planets are packed so tightly—the farthest planet is just one-sixth the distance from the star as Mercury is from the Sun—that TRAPPIST-1 would loom large from the vantage point of any planetary resident. Such close quarters also ensures short orbital periods, on the order of days, to maximize opportunities for syzygy. Next, none of the planets are puny, so they serve as good obstructers; the smallest, TRAPPIST-1d, is 77% the size of Earth. Best of all for eclipse hunters, the seven planets orbit on roughly the same plane; that’s how researchers were able to detect them via the transit method earlier this year . That means an eclipse occurs every time a planet passes between the star and an observer; there’s no need to worry about orbital inclinations the way Earthlings have to with the Moon.

Combining all those factors leads to a manic set of eclipses in the TRAPPIST system. Take the view from planet e, which astronomers consider the most potentially habitable of the seven worlds. From here TRAPPIST-1 would appear more than four times as wide as the Sun does from Earth. Every two days planet b, with a radius 60% that of the Moon as seen from Earth, would cross in front of the star. Planet c would do the same every four days and planet d every 12 days. Frequently two or even three of the inner planets would transit at the same time, creating an incredible sight. “There would be black spots crossing the sun all the time,” says Robert Hurt , a visualization scientist with NASA’s Spitzer Space Telescope who calculated the apparent sizes of the system’s constituents from various vantage points. (Disclaimer: All of this assumes that the viewer is on the day side of the planet, which is almost certainly tidally locked with the star. Sky watchers chilling on the night side would have to settle for lunar eclipse analogs in which planet e casts its shadow on the outer planets.)

The one experience the TRAPPIST system does not afford, at least as far as we know now, is a total solar eclipse. Because of the tight orbital packing, the star simply appears too large in the skies of all the planets to ever be blocked completely. Still, the possibility of totality emerges if it turns out that any of the TRAPPIST planets have moons. Despite several years of analysis of Kepler data, researchers have yet to find a definitive signal of a moon orbiting any exoplanet. But if our solar system is any indication, exomoons are out there, stabilizing their planetary companions, generating tides, and if the geometry is right, producing eclipses.