The wild wild worlds: a guide to the weirdest planets in the Milky Way – Astronomy Magazine

The Planet that Acts Like a Comet

Planets typically orbit their host stars in rounded ellipses, while comets follow long, narrow orbits that carry them far out into the cold reaches of the solar system before falling inward again. HD 20782bs orbit looks more like that of a comet than a giant planet twice as massive as Jupiter. The gas giant slingshots around a G-type main sequence star 117 light years away in the constellation Fornax, swooping in just 5.5 million miles from the star (seven times closer than Mercurys orbit of our Sun) before making a long swing 232 million miles out into its solar system (about the distance from the Sun to the Asteroid Belt). That gives HD 20782b an orbital eccentricity of 0.96: its path through space is a long, narrow ellipse, not the round, nearly circular kind that most well-behaved planets follow.

That wild orbit is probably thanks to a series of gravitational pushes from another gas giant orbiting the same star (which astronomers havent spotted yet), or possibly from the other star in its binary system, HD 20781. In fact, this is the first binary system astronomers have found where both stars have their own planets.

HD 20782b swoops past its star too fast for the stellar wind to blast away much of the gas giants atmosphere, so despite its daringly close perihelion, the planet still has clouds of icy particles, like the ones in Jupiters upper cloud layers. Starlight reflecting off those icy clouds allowed astronomers to learn more about the planet in 2016, ten years after changes in the stars radial velocity first revealed the planets existence.

A Planetary Family Feud

A long time ago in a solar system 1,200 light years away,, two gas giants collided and flung each other to the far ends of the solar system. CVSO 30b, detected in 2012, orbits the young M3 star CVSO 30 at just 1.2 million kilometers, a tiny fraction of the distance between Mercury and the Sun. Thats right on the edge of its Roche limit, the distance at which the stars gravity will start to rip the planet apart. In fact, CVSO 30b may already be close enough for its host star to start stripping away its mass. It takes just 11 hours for the gas giant to complete an orbit.

At the other end of the solar system, its sister planet CVSO 30c, detected in 2016, keeps its distance with a 99 billion kilometer orbital radius, taking 27,000 years to make a lap around the star. CVSO 30 is a fairly small star, less than half the mass of our Sun, so its unusual to find two super-sized gas giants caught in its gravitational pull. In fact, CVSO 30c is so large that its discoverers say its probably a type of brown dwarf, too large to be a proper planet and too small to become a star, hovering awkwardly on the threshold.

And given that the two planets are pretty close in mass (each is around 5 times as massive as Jupiter) their orbits shouldnt be so wildly different, according to most models of how solar systems form. In fact, this is the first solar system astronomers have ever seen in which two planets have such different orbits.

CVSO 30c probably didnt start its life so far from its parent star. In fact, it probably formed in a position more like the one Jupiter occupies in our own solar system, but at some point in the solar systems history, 30b and 30c interacted gravitationally and flung each other into their current extreme orbits, in what astronomers described in a 2016 paper as a mutual catastrophic event of planet-planet scattering. Its probably not stable in the long run but for purposes of astronomers here on Earth, its close enough. That gives astronomers a rare opportunity to study what happens when gas giants interact.

The system may not be what it appears, however. A 2015 paper suggested that CVSO 30b might not exist at all. If thats the case, CVSO 30c is so far out from CVSO 30 that it may not actually be orbiting the star at all. It could be a free-floating object in space, which isnt uncommon for objects of its mass and type. So far, astronomical observations havent been able to confirm that the giant planet and the nearby star are actually moving regularly in relation to one another, so its possible that theyre simply not. On the other hand, astronomers say thats highly unlikely, with odds on the order of .00002.

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The wild wild worlds: a guide to the weirdest planets in the Milky Way - Astronomy Magazine