About 57 light years from Earth, astronomers have discovered a large new planet, colored a deep magenta. It’s the second planet whose color has been directly observed by astronomers, the first being HD 189733b. That alone would make this find noteworthy. But equally noteworthy is the fact that the planet itself challenges current theories of planetary formation.
This planet, GJ 504b, is about the size of Jupiter, but has several times its mass. It’s actually so far the smallest planet that’s ever been directly imaged with a telescope, rather than being observed by eclipsing its parent star.
Planets the size of Jupiter appear to be quite common throughout our galaxy – astronomers have discovered many outside of our solar system. But what’s unusual about this planet is that it’s located about 4.05 billion miles from its star – about 43.5 times the distance between the Earth and the Sun. If it were in our own solar system, it would be beyond the orbit of Neptune.
That’s what poses a problem for astronomers. The current model for how Jupiter-sized planets form is called core accretion theory. In this model of planetary formation, after a star is formed, it’s surrounded by a massive field of debris. At some point, comets or asteroids in the field collide, producing a more massive body. That body then collides into other bodies and gets more massive.
Eventually, as the body gets more massive, its gravity starts attracting dust and gasses in the debris field. Like a snowball, the planet gets bigger and bigger as it attracts more mass. Eventually, a gas giant is formed.
But here’s the thing – the orbit of Neptune is about at the outer edge of distances from a star that such planets could form. When they’re out further, the debris is estimated to be too dispersed to form a planet as massive as Jupiter, because there’s just not enough stuff out there to accumulate.
And yet, there’s GJ504b, sitting at a distance much further away from its star than Neptune does ours.
“This is among the hardest planets to explain in a traditional planet-formation framework,” researcher Markus Janson said in a press release. “Its discovery implies that we need to seriously consider alternative formation theories, or perhaps to reassess some of the basic assumptions in the core-accretion theory.”
Earlier this year, astronomers using the Hubble Space Telescope found what they believe to be a planet around the star TW Hydrae. That planet, too, exists further out from its parents star and is much younger than current planetary formation theories would predict.
Thanks to these observations, astronomers are learning a lot more about planets outside of our solar system. And in the process, they may end up having to re-think how those planets developed.
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