Mystery: Is our solar system normal?
ON THE face of it, it’s a biggie: fully 1.4 times the diameter of Jupiter, the largest planet in our solar system. But it is nothing unusual in itself. Discovered in 2016, KELT-11b at first seemed to be just one of many large “hot Jupiters” orbiting close to their star, causing a large drop in light whenever they cross its face.
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But when astronomers calculated KELT-11b’s mass, they found it was a minnow, with just 20 per cent of Jupiter’s mass. Combine that figure with its size, and the average density of the planet is little more than that of polystyrene packaging.
We now have a growing roster of these puffy planets: just this year we found WASP-127b, which has very similar vital statistics to KELT-11b. The problem is that they fly in the face of everything we thought we knew about planet formation, based on our solar system. “We don’t really understand how they get so inflated,” says of Lehigh University in Pennsylvania, who led the KELT-11b team.
Our solar system makes sense to us. There are the small rocky planets including Earth close in, and gas giants such as Jupiter further out. When the planets were forming, the heat near the sun chased off most gas, but greater quantities of volatile substances condensed in the cooler, outlying regions, providing bigger solid cores around which vast balls of gas accumulated. However, the puffballs and various other oddballs we have found show this is far from always the case.
The only lead we have to go on is that the oddities are always very close in to their star. “It certainly seems to be related to the level of radiation they are getting,” says at the University of Warwick, UK, who helped discover WASP-127b. But calculations show that is still not enough to account for their inflation.
Hot Jupiters and puffy planets are all thought to form further out and migrate inwards. All planets are hot early on, as the clumping together of the debris that forms them releases gravitational and kinetic energy. If they migrate before they can cool off, perhaps the environment by the star is so toasty that they can never shed excess heat. Or perhaps the planets get reinflated, possibly by particle winds or magnetic fields from the stars.
No one truly knows. “When you think about it, it is incredible,” says Pollaco. “Something is working incredibly efficiently to inflate those planets.”
The more planetary oddities we see, the more we must confront a wider question: are they the rule, and our solar system the exception? Again, the jury is out. “We don’t know if our solar system configuration is common or rare,” says Pepper. “We just simply haven’t been able to probe enough systems in enough different ways to know that.”
This article appeared in print under the headline “Object: Planet Kelt-11B”
