Many of the exoplanets we've discovered look at least vaguely like something we're familiar with. Exoplanets have been described as super-Earths, mini-Neptunes, hot Jupiters, and so on. But not everything is entirely familiar, and we've stumbled across a number of oddballs. Among those oddballs is a group of planets with extremely low density. Lacking a Solar System analogy has forced us to come up with a non-planetary nickname: the super puffs.

Many of the super puffs are a bit hard to explain via normal planetary physics. But a group of researchers based in Europe looked at a possible alternate explanation for one super puff: it's a normal planet with unusual rings. The answer they came up with is that we can't really tell right now, but they do suggest ways that we could possibly sort this all out.

More than just a puff

Although intense pressures make the cores of gas giants quite dense, on average, they're still gas, and they often have a large percentage of lighter gasses like hydrogen and helium. This, famously, has led to the contention that if you could somehow drop Saturn onto some water, it would float. (It would actually be torn to shreds by the gravitational pull of that much water, but we mostly let that detail slide.)

Super puffs make Saturn look like a heavyweight. Water's density is a gram per cubic centimeter. Saturn's density is 0.7 g/cm3. A super puff can have a density in the neighborhood of 0.1 g/cm3.

And that creates some issues, physics-wise. At that point, the matter gets very diffuse, and it's tough to find ways to have enough mass to hold it all together. There are some planets that are close enough to their host stars that they've likely heated up and expanded their atmosphere accordingly. This isn't stable—these planets will probably lose their atmospheres over time—but the process is slow enough that we'll likely be able to catch them in the middle of it.

Another possible explanation is a combination of errors on our part. The density of anything is simply the mass of the object divided by the volume it occupies. We figure out the mass of a planet through its gravitational interactions with its host star. We figure out the volume by looking at how much light the planet blocks as it passes in front of its host star. There are errors in both of these and the potential for complications like planets elsewhere in the exosolar system that we've not identified yet. It's possible that some of the super puffs are a bit closer to normal planets, and we've just been a bit off in our measurements.

Inflated at the HIP

The new work focuses on a super puff, HIP 41378 f, an extremely low density, Saturn-sized planet. It's an interesting super puff because at least one explanation for its low density can be ruled out: The planet orbits its star at a greater distance than the Earth orbits our Sun (it takes 542 days to complete an orbit). While HIP 41378 f is slightly brighter than the Sun, it's nowhere near bright enough to vastly inflate an atmosphere at that distance.

But the measurement problem could easily be an issue, as we have detailed data from only two transits. While there's a lot more data on the planet's mass, it remains possible we're a bit off on the volume calculations. That said, the researchers take the values we have and try to calculate the relative likelihood of two scenarios: one in which it's an extremely weird planet, and the other is that it's a smaller planet with rings.

These probabilities are bit hand-wavy, given that all options seem to be pretty unlikely. To get a super puff planet like this, you have to have a dense core surrounded by an extremely tenuous atmosphere. But an atmosphere that tenuous isn't going to be opaque enough to blot out as much starlight as HIP 41378 f does, so you have to assume there's some process adding opacity. Clouds won't do it, because there's probably not enough material in the atmosphere to form clouds. So you have to imagine a process sending dust from the planet's surface into the upper atmosphere—which has to be extremely distant from the planet's surface to get something this big.

All of which makes a single spherical planet pretty unlikely, which you might think makes things favorable for a ringed planet. But that's only because we haven't gotten into the issues there.

Put a ring on it

For the ring to work, it has to be in a plane that's not coincident with the plane of the planet's orbit. That's certainly possible, but that orientation would mean the gap between the planet and the ring should be oriented to allow starlight through at various points during its orbit. This should create some anomalous light leaks in the middle of its transit. To the best of the researchers' ability to tell, none of those were there. But with only two transits worth of data, it would have been difficult to spot them anyway.

Even so, this demands what's essentially an impossible situation: the rings have to be dense enough to minimize the light getting through, while extending to nearly the planet's surface for the same reason. Those two situations are largely incompatible.

The one thing that does look good about the ring model is that the radius of the planet drops from nine times Earth's radius to only 3.7 times. This raises its density to higher than Saturn's and higher than water, placing it in the neighborhood of Uranus'. That, at least as far as the physics of the planet itself goes, is pretty reasonable.

In the end, the researchers conclude that there's not enough data to favor either option at this moment, and even trying to do probability calculations requires them to make a lot of assumptions that aren't really justified by the data we have at the moment. So, naturally, their solution is to get more data. They note that further observations to constrain the mass of the planet are already planned.

But they suggest the most interesting thing we can do is observe a transit using the far infrared end of the spectrum. In this area, the photons should largely come straight through the dusty material in the disk. Thus, if a disk is responsible here, the transit should be greatly reduced compared to its depth at other wavelengths. Unfortunately, given that there's over a year and a half between transits, we'll have to be patient on that one.

Astronomy & Astrophysics, 2020. DOI: 10.1051/0004-6361/202037618 (About DOIs).