The cotton-candy planet

There's a general pattern you see in astrophysics, which arises from the fact that gravity is both (1) always attractive, never repulsive, and (2) extremely weak.

It's hard to overstate the "extremely weak" bit.  The next strongest of the four fundamental forces, electromagnetism, is 36 orders of magnitude stronger; that is, the electromagnetic force is 1,000,000,000,000,000,000,000,000,000,000,000,000 times more powerful than gravity.  This may seem odd and counterintuitive, since the gravitational pull on your body seems pretty damn strong (especially when you're tired).  But think about it this way; if you use a refrigerator magnet to pick up a paper clip, that little magnet is able to overcome the force of the entire Earth pulling on the clip in the opposite direction.

The practical result of these two features of gravity is that at small scales and low masses, the effects of gravity are essentially zero.  If I'm picking up a book, I don't have to adjust for the negligible gravitational attraction between myself and the book, only the attraction between the book and the enormous mass of the Earth.  On the largest scales, too, the effects of gravity more or less even out; this is called the flatness problem, and is something I dealt with in more detail in a recent post.  (Plus, on these cosmic scales, the force of expansion of spacetime itself -- something that's been nicknamed dark energy -- takes over.)

It's at mid-range scales that gravity becomes seriously important -- objects the size of planets, stars, and galaxies.  And there, the other feature of gravity kicks in; that it always attracts and never repels.  (Whatever Lost in Space may have had to say about anti-gravity, there's never been evidence of any such thing.)  So for objects between the size of planets and galaxies, gravity always wins unless there is some other force opposing it.

This, in fact, is how stars work; the pull of gravity from their mass causes the matter to collapse inward, heating them up until the fusion of hydrogen starts in the core.  This generates heat and radiation pressure, a balancing force keeping the star in equilibrium.  Once the fuel runs out, though, and that outward force diminishes, gravitational collapse resumes -- and the result is a white dwarf, a neutron star, or a black hole, depending on how big the star is.

All of this is just a long-winded way of saying that if you've got a mass big enough to form something on the order of a planet or star, it tends to fall inward and compress until some other force stops it.  That's why the insides of planets and stars are denser than the outsides.

Well, that's how we thought it worked.

The latest wrench in the mechanism came from the discovery of a planet called WASP-193b orbiting a Sun-like star about 1,200 light years away.  On first glance, WASP-193b looks like a gas giant; its diameter is fifty percent larger than Jupiter's.  So far, nothing that odd; exoplanet studies have found lots of gas giants out there.

But... the mass of WASP-193b is only one-seventh that of Jupiter, giving it the overall density of cotton candy.

So I guess in a sense it is a gas giant, but not as we know it, Jim.  At an average density of 0.059 grams per cubic centimeter, WASP-193b would float on water if you could find an ocean big enough.  Plus, there's the problem of what is keeping it from collapsing.  A mass one-seventh that of Jupiter is still an impressive amount of matter; its gravitational pull should cause it to pull together, decreasing the volume and raising the density into something like that of the planets in our own Solar System.  So there must be something, some force that's pushing all this gas outward, keeping it... fluffy.  For want of a better word.  

But what that force might be is still unknown.

"The planet is so light that it's difficult to think of an analogous, solid-state material," said Julien de Wit of MIT, who co-authored the study, in an interview with ScienceDaily.

[Image licensed under the Creative Commons NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani, Artist impression of ultra fluffy gas giant planet orbiting a red dwarf star, CC BY 4.0]

"WASP-193b is the second least dense planet discovered to date, after Kepler-51d, which is much smaller," said Khalid Barkaoui, of the Université de Liège's EXOTIC Laboratory and first author of the paper, which was published in Nature Astronomy last week.  "Its extremely low density makes it a real anomaly among the more than five thousand exoplanets discovered to date.  This extremely-low-density cannot be reproduced by standard models of irradiated gas giants, even under the unrealistic assumption of a coreless structure."

In short, the astrophysicists still don't know what's going on.  Twelve hundred light years from here is what amounts to a planet-sized blob of cotton candy orbiting a Sun-like star.  I'm sure that like the disappearing star from my post two days ago, the theorists will be all over this trying to explain how it could possibly happen, but thus far all we have is a puzzle -- a massive cloud of matter that is somehow managing to defy gravity.

As Shakespeare famously observed, there apparently are more things in heaven and earth than are dreamt of in our philosophy.

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