Family of Four Planets Questions Formation Theory

12 December 2010


Observations made by the Keck telescope in Hawaii have confirmed a fourth planet orbiting a nearby star but have thrown current theories of planetary formation into doubt.

HR 8799Publishing in the journal Nature this week Christian Marois, from the Herzberg Institute of Astrophysics in Canada, and colleagues in the United States analysed near infra-red images of the star HR 8799 and found a new exoplanet to add to the three previously discovered.  These planets have particular properties that make this sort of imaging possible:  They're very massive, the new one is somewhere between 7 and 10 times the mass of Jupiter; they're a long way from the star itself, over 25 times the distance between the Sun and the Earth; and they're relatively young, less than 100 million years.  Younger planets are still hot from their creation, which makes them glow brightly in these frequencies.

Finding new planets is always fascinating in itself but the really interesting thing about this system is that it doesn't fit with current models of planetary formation.  There are 2 models that describe the formation of giant planets like this. There's the core accretion model, where small bits of rock collide and gradually clump together until the planet has enough mass and, therefore, gravity to hold on to an atmosphere. Then there's the disc instability or the gravitational collapse model, where variations in the disc of material around the star, the protoplanetary disc or proplyd, will condense out into balls of gas that gradually collect dusty matter at the core.

In the case of the HR 8799 system, the innermost planet could have formed by core-accretion, but those further out would need far longer; in fact, more than the age of the star itself.  However, If we look to the disc instability model, the protoplanetary disc would not have been in the right condition to allow the innermost planet to form when it did.

So how did these planets get there?  Right now, we can't be sure.  The authors say that "a hybrid process with different planets forming through different mechanisms cannot be ruled out, but seems unlikely with the similar masses and dynamical properties of the four planets."  The other option is that the planets formed elsewhere and then moved into their current locations, but until we find more systems like this one we simply don't have enough information to make a decision.  But, with exoplanets being discovered at an astounding rate, we may find more of these planetary families soon.


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