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Astronomers Found Jupiter-Sized 'Cousin' Planets for the First Time

Hot Jupiters orbiting twin stars prove that hey, that can happen.

Mars is our brother planet and Venus is our sister planet, but what the heck is a cousin planet? Hint: we don't have one, because our Sun is an only child.

But for the first time ever, astronomers have discovered an extended stellar family of Jupiter-sized planets, each orbiting a different twin star in a binary ("twin") system. The find could shed literal light on the atmospheres of so-called "hot Jupiters," and help explain why planets in this category end up wandering inward to occupy tight orbits around their host stars.

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The solar family was discovered by the WASP (Wide Angle Search for Planets) consortium, an international group of astronomers operated jointly by Keele University and  the University of Warwick in the UK. A Swiss branch of the team based in South Africa was observing the star WASP-94A, and noticed that its luminosity regularly dimmed, suggesting a large exoplanet was transiting in front of it and periodically blocking its light.

The discovery of the second exoplanet, orbiting WASP-94B, was pure serendipity. "We observed the other star by accident, and then found a planet around that one also!" said lead author Marion Neveu-VanMalle of Geneva Observatory, in a Keele University statement. Neveu-VanMalle and her colleagues published their findings in the journal Astronomy & Astrophysics on Monday.

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Hot Jupiters are a weird case study in the ever-growing pantheon of exoplanet types. They form way out in the solar boonies as our local hero Jupiter did, but for some unknown reason, they get restless and slowly migrate towards the center of their systems. Hot Jupiters end up orbiting 0.15 to 0.50 astronomical units away from their stars, at least ten times closer than Jupiter is to the Sun at 5.2 astronomical units.

It's a good thing that Jupiter never felt the urge to share this destiny, because apparently hot Jupiters can kick terrestrial, Earthlike planets clear into interstellar space on their journey towards the light.

On the other hand, any terrestrial planets that do manage to form in the wake of a hot Jupiter will end up being particularly water-rich, according to a 2007 ESO study. So the pros and cons of having a big gas giant plow on through the inner solar system may balance out in the end.

The WASP-94 system provides a great opportunity to study these rare and bizarre exoplanet dynamics, as well as their repercussions on the development of habitable worlds in their systems. It may be that hot Jupiters rely on the unique gravitational interplay of twin stars to pull them into tighter orbits, but that theory is complicated by the existence of hot Jupiters like 51 Pegasi b, which orbits a solo Sun-like star.

The cousin planets may also be the perfect test ground for studying the atmospheres of hot Jupiters, given that there are two high-watt light sources shining down on them. "WASP-94 could turn into one of the most important discoveries from WASP-South," said study co-author Coel Hellier in a statement. "The two stars are relatively bright, making it easy to study their planets, so WASP-94 could be used to discover the compositions of the atmospheres of exoplanets."

The study is yet another example of the spectacular diversity of solar families that have been detected by exoplanet-hunters. With advanced space telescopes like the JWST, TESS, and PLATO in the works, the discoveries are only going to accelerate from here.