Puff piece

A couple of posts ago I mentioned how there have been discoveries of exoplanetary systems that have challenged our understanding of how planets form -- including planets with wildly misaligned orbits and the phenomenon of "hot Jupiters," gas giants close enough to their parent stars that you'd expect (based on what we understand of the physics) they would have their lightweight, hydrogen-rich atmospheres blown clean away.

Yesterday I ran into some new research showing that the latter phenomenon -- low-density atmospheres being oddly resistant to annihilation -- is a problem with another kind of exoplanet, which are called (I shit you not) "super puff planets."  They are not, I hasten to point out, where these characters come from:

Although admittedly, those three do look like aliens to me.  I mean, what's with the ginormous eyes, and the fact that they appear not to have noses or fingers?  And apparently they can fly, although I only know this by inference from the still-shots, as I have never watched the show (and have no intention of doing so).  A few years ago I was maneuvered by a friend into watching a forty-five second clip from My Little Pony, and that did enough psychological damage that I'm wary of walking into the same danger again.

But I digress.

"Super puff planets" aren't fiction at all; they are planets with an extraordinarily low density.  The lowest-density planet in our own Solar System is Saturn, which at an average of 0.69 g/cm^3 is light enough that it would float in water (if you could find a pool big enough).  But super puff planets beat Saturn by a mile; they have an average density of 0.05 g/cm^3, which is about the same as cotton candy.

The open question, of course, is not only how they don't get blown apart by the light, heat, and stellar wind from their parent stars, but what allows them to form in the first place.  Planets are held together because they're massive enough that their gravitational energy overcomes other forces (like electrostatic repulsion) that might act to fragment them.  This is why planets are all roughly spherical; it's the equilibrium shape for something that is gravitationally bound -- and the heavier they are (like the neutron stars that were the subject of my post a couple of days ago), the rounder they are.  Small bodies, like the asteroids and some of the smaller moons of the planets in our Solar System, can be some other shape; massive bodies are pulled into spheres.

So how super puff planets (1) form, and (2) don't get immediately torn to shreds, is still unknown.  Which makes it even wilder that a paper this week in The Astronomical Journal describes a system, Kepler 51, that has three -- perhaps four -- of these cotton-candy planets.

"Super puff planets are very unusual in that they have very low mass and low density," said Jessica Libby-Roberts, Center for Exoplanets and Habitable Worlds Postdoctoral Fellow at Penn State, and co-first author of the paper.  "The three previously known planets that orbit the star, Kepler-51, are about the size of Saturn but only a few times the mass of Earth...  We think they have tiny cores and huge atmospheres of hydrogen of helium, but how these strange planets formed and how their atmospheres haven't been blown away by the intense radiation of their young star has remained a mystery.  We planned to use JWST to study one of these planets to help answer these questions, but now we have to explain a fourth low-mass planet in the system!"

Explanations, thus far, have proven elusive.  Right now, super puffs and hot Jupiters are both mysteries, planets that somehow form and maintain a thick atmosphere when by everything we know of astrophysics, they should be more like airless, rocky, superheated Mercury.  It does raise one hopeful thought, however; one of the "Goldilocks zone" qualifications for planetary habitability is that the planet in question needs to resist having its atmosphere blown away by the stellar wind of the parent star.  M-type red dwarf stars, for example, by far the most common stars in the galaxy (they make up an estimated seventy percent of the stars in the Milky Way), have been thought by some to be ruled out as habitable systems on that basis.  First, being low-temperature, any planets warm enough to have liquid water would have to be in close orbits; and second, those close orbits would make the planet a target for stellar storms, potentially destroying any atmosphere the planet had.

This may need to be rethought.  Atmospheres, as it turn out, may be a great deal more durable than we'd reckoned.

So there's yet another odd and unexplained phenomenon from the skies.  Planets as light as cotton candy.  Shakespeare knew whereof he spoke when he wrote, "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."

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