Triple Play: A planet with three suns

From the vantage point of a newly discovered planet 149 light-years from Earth, it isn’t hard to look on the bright side: Three suns grace the skies above this hot, Jupiterlike body. The planet tightly orbits the main star of a closely-knit trio of stars collectively known as HD 188753.

SUNS UP. View from a hypothetical moon that might orbit a just-found extrasolar planet residing among a trio of closely orbiting stars. In this illustration, the three stars, one setting, grace the sky. R. Hurt/Caltech

A planet among a trio of stars isn’t merely a novelty. The orb’s very existence poses a challenge for how planets form, discoverer Maciej Konacki of the California Institute of Technology in Pasadena reports in the July 14 Nature.

Although binary and multiple-star systems are more common in the Milky Way than single stars such as our sun are, many researchers had assumed that the mutual gravity of a trio of closely orbiting stars would prevent planets from growing as big as Jupiter. That’s because the raw material for planets comes from a swirling disk of gas, dust, and ice that surrounds a newborn star. In the standard recipe for making such a massive planet, a solid core about 10 times the mass of Earth coalesces from this protoplanetary disk. The core then captures a vast envelope of hydrogen and helium gas until it becomes as hefty as Jupiter, about 300 times as heavy as Earth.

According to this scenario, the newly discovered planet—as well as the 30 or so other Jupiter-mass bodies that lie within roasting distance of their stars—probably formed much farther out, in an icy part of the disk where it’s relatively easy for a large planetary core to form. That suggests the newfound planet coalesced in a region at least three times the Earth-sun distance from the main star in HD 188753.

However, all three stars lie as close to each other as Saturn’s distance from the sun, suggesting that their gravity would have stripped away the outer, icy part of the primary star’s disk. The truncated disk would have a radius only slightly bigger than the Earth-sun distance, providing no room for a Jupiterlike planet to form. That would seem to leave no explanation at all for the planet.

In a commentary accompanying the July 14 Nature report, a pair of researchers suggests several ways out of this conundrum. The planet-making material that once surrounded HD 188753 might have been unusually thick, allowing for the growth of a Jupiterlike body even within a truncated disk, propose German astronomers Artie P. Hatzes of the Thuringia State Observatory and Günther Wuchterl of the Astrophysical Institute and the University Observatory of the Friedrich Schiller University in Jena. It’s also possible, says Hatzes, that some ice can exist within the inner, warmer part of a protoplanetary disk.

Whatever the explanation, say Hatzes and Wuchterl, the new finding suggests that the often overlooked population of binary-and triple-star systems throughout the galaxy might harbor a plethora of planets.

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