New alien world challenges accepted theories on star system formation

Astronomers have plenty of evidence of planets orbiting around other stars. In a small number of cases, they actually have direct images of these planets, and not only have they now added one more to the list, but it's also a breakthrough — the lowest-mass planet ever imaged around a star like our Sun, and so far from its star that it challenges accepted ideas of how planets form.

Detecting planets orbiting around other stars is tough. Stars are so bright and so distant that any planets orbiting around them are usually completely lost in images of the star (picture trying to see a speck of dust stuck to a light bulb, while standing on the other side of the room). As a result, astronomers have come up with some very innovative ways of finding them. They watch for 'transits', where a star's light dims as a planet passes in front of it. They watch for distortions of light due to the 'gravitational lensing' effect of planets. They look for 'wobbles' seen in a star due to the effect of a planet orbiting around it. However, there are some cases where a planet is large enough and distant enough from its star that we can actually see it in pictures.

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In this case, the star in question is GJ 504, a yellow star like our Sun (but about 30 times younger) on the outskirts of the constellation Virgo. The planet is 'GJ 504b' — a gas giant world roughly the same size as Jupiter, but four times Jupiter's mass — which orbits its star roughly 43.5 times the distance from Earth to the Sun.

It was infrared images from the Subaru Telescope, at the summit of Mauna Kea, Hawaii, that spotted the planet.

One interesting thing about this discovery is that it challenges some of the ideas in place for how planets like Jupiter and GJ 504b form. In the 'core accretion model' for gas giants, the planets first form a rocky core (similar to how planets like Earth form), and then they gather more and more gases to them from the star's protoplanetary disk, building up to their immense size and pressure. However, this model only works for planets forming out to around 30 times the distance between the Earth and the Sun. With GJ 504b forming much further out, it's going to take some work to figure out how that happened.

"This is among the hardest planets to explain in a traditional planet-formation framework," said Markus Janson, a Princeton University postdoc who was on the discovery team, according to the statement. "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."

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With our own solar system as the only example of a star system we've had up until recently, physicists have developed some very solid theoretical models for how our solar system formed. As we discover more and more planets and star systems out in the galaxy, and see the diversity that is actually possible, there are likely many parts of those models that are going to need tweaking to account for it all. The science is still solid, but finds like this just show how planet hunting is one of the most exciting branches of astronomy and astrophysics these days.

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