Second stars can distort planet size estimates

by Sarah Derouin
Wednesday, October 18, 2017

An artist's illustration of a two-star system: One star can be obscured by the light of another, making it difficult to estimate the size of planets orbiting the stars. Credit: NASA/JPL-Caltech.

When light from a star is blocked by another celestial body — as when the moon obstructed light from the much-larger sun during the recent solar eclipse — astronomers can estimate the density of exoplanets that orbit the star. And from the density, they can determine whether a planet is rocky like Earth or gaseous like Jupiter. But a new study in the Astronomical Journal shows that such density assessments, which are normally calculated from the size of the planet, may be skewed by the presence of “hidden” stars.

Light from faraway stars can be hard to discern through a telescope — imagine trying to recognize individual street lights from space. Binary star systems, featuring two stars that orbit each other, can appear as a single point of light, even through the powerful lens of NASA’s Kepler Space Telescope. Now, new high-resolution imaging has revealed that some systems previously thought to have one sun-like star actually have two. Having two light sources, it turns out, muddies planetary density calculations made using the traditional blocked-light method, which assumes a single source.

Depending on which star in a binary system a planet orbits, the size, and thus the density, that astronomers estimate for that planet can vary. If a planet orbits the brighter of two stars, light from the dimmer star is unlikely to alter density estimates made using the traditional method. But if two paired stars are equally bright, actual planetary sizes can be about 40 percent larger than originally estimated; the corresponding density, meanwhile, would be less than estimated. If a planet orbits the fainter of two stars, it could actually be two to three times larger, or more, than if the size is determined without accounting for the double dose of starlight.

In the new work, Elise Furlan of Caltech and Steve Howell of NASA’s Ames Research Center found that 43 of the 50 planets previously identified by the Kepler telescope had blocked, or occulted, light from a second star that needed to be factored into size calculations. For 35 out of the 50, accounting for the second stars did not significantly change existing size estimates. For the remaining 15, the researchers could not tell whether the planets orbited the brighter or fainter star in a binary pair, but five of the planets in this group are in systems where the companion stars are of roughly equal brightness. For these five, the team noted, “the decrease in density would be substantial” no matter which star each orbits.

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