Scientists say they may have a clue to the mysterious origin of "hot Jupiters," those gas giant planets that circle uncomfortably close to their host star: There may be a second star involved. A survey of 77 stars hosting hot Jupiters has found that roughly half of them are binary star systems.
The findings, presented at the Canadian Astronomical Society's annual meeting in Winnipeg, shed light on the dynamics that shape these strange planetary systems – and might even offer some insight into our own solar system.
Ever since hot Jupiters were discovered more than two decades ago, scientists have wondered how these star-scorched worlds came to be. After all, in our own neighborhood, the gas giants lie far from the sun, in the frigid outer solar system.
"One big mystery about these hot Jupiters is, how did they get in such orbits?" said lead author Henry Ngo, a PhD student in planetary science at Caltech.
Several theories have said there may have been a "friend" – either a nearby star or another massive planet – that could have influenced either the hot Jupiter's orbit or its formation.
Ngo and his colleagues set out to find some of these "friends." They surveyed 77 star systems that had hot Jupiters circling them, using the Keck Observatory on Mauna Kea in Hawaii to look for any stellar companions. Sure enough, about half of them (47%, give or take 7%) had companion stars within a distance of 50 to 2,000 astronomical units.
Keep in mind, an astronomical unit, or AU, is the average Earth-sun distance, or about 93 million miles. For comparison, Pluto's average distance from the sun is roughly 40 AU; the distance to the star closest to the sun, Proxima Centauri, is about 268,000 AU. So in interstellar terms, 50-2,000 AU is practically spitting distance.
"It's a clue that maybe binary systems create environments that are easier for hot Jupiters to form," Ngo said.
The researchers did find something surprising: There didn't seem to be a correlation between the existence of a companion star and the misalignment of the scorched gas giant's orbit with its parent star. Well-aligned and misaligned hot Jupiters had roughly the same likelihood of having a companion star nearby. That means that a theory known as the Kozai-Lidov mechanism, which describes how an object's orbit might be perturbed by a third party, might not be the main mechanism causing these hot Jupiters after all. In fact, it might account for the existence of fewer than 1 in 6 hot Jupiters, Ngo said.
"That confused us for a bit, because that kind of ruled out one of the main theories," he added.
In any case, these companion stars could potentially be close enough to influence the emergence of these hot Jupiters, whether they were born in place or pulled close to the star later. What exactly such mechanisms might be remains unclear for the moment, the researchers said.
It's possible, for example, that a binary star system would have had a larger protoplanetary disk, which would allow more planets (such as a hot Jupiter) to coalesce.
"We are thinking of ways of testing whether or not that idea is correct," Ngo said.
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