I just received another copy of my book Under Alien Skies translated into a foreign language: Chinese!

Yay! 1.4 billion potential new readers!

This makes 4 foreign editions for the book: Italian, French, Estonian, and Chinese. I’m really thrilled it’s making its way across this planet. Maybe eventually other ones, too.

We’ve discovered all manners of weird planets orbiting other stars, which I suppose can be expected when there are nearly 6,000 to choose from. Some would look right in place in our own solar system, and others are truly bizarre. 

One weird flavor of planet are the puffy Jupiters, also called puffballs, or even cotton candy planets.

These are planets that are similar in mass to Jupiter, but far larger in size — this in turn means they are very low density. Another way to think of them is being inflated; something is causing these planets to puff up.

A big hint comes from their temperature: these planets tend to orbit very close to their stars. Planets this size have very deep atmospheres, and if they’re close to their host stars the incoming heat makes the gas expand, just like in a hot air balloon. The planet puffs up.

Elina Zhang, an astronomer at the Institute for Astronomy, found a couple of exceptions to this rule, though. Going through the data from TESS, the Transiting Exoplanet Survey Satellite, Zhang discovered two planets that are puffy, but, crucially, they’re not hot!

The planets are called TOI-7018.01 and TOI-7081.01 (TOI stands for TESS Object of Interest, for stars that have planet candidates). Both orbit stars pretty similar to the Sun, and while both also orbit their stars relatively close in (closer than Mercury is to the Sun), they’re both well below the sort of temperature you’d expect for inflation — 400 and 650°C (I’ll note these temperatures are not measured but instead inferred from their distances to their host stars).

Still, both are bigger than you’d expect, about 1.6 times the size of Jupiter. That may not sound like much, but volume goes up with the radius cubed. Both are four times the volume of Jupiter! Plus, due to weird quantum physics that happens in the cores of such planets, Jupiter is about as big a planet you can get; dump more mass into it and it would actually get smaller. So it’s really strange they’re bigger than you’d expect.

Zhang looked into ways this could be a mistake. TESS finds planets when their orbits are edge-on as seen from Earth; when the planet passes in front of the star we get a mini-eclipse, and the amount of light we see from the star drops. If we know the size of the star then the amount of the dip tells us the size of the planet. But Zhang found no unusual things going on that might throw off the calculation (like a small faint companion orbiting the star, or the star being binary). From the data, it really does look like what we have here are a pair of cold puffy Jupiters.

So what’s inflating them? That’s not clear. Planets are hot when they form, and that heat leaks out over billions of years. It’s possible the atmospheric composition or structure of these two planets has done a better job insulating their interiors, so they’re hotter than expected and therefor puffier. As I mentioned above we don’t know the actual temperatures of these planets, so maybe they’re warmer than what you’d think given how close they are to the stars. 

So here we are, with nearly 6,000 planets under our belts, but we’re still finding ones that defy expectation or (at least for now) explanation. On the other hand that’s 6,000 out of hundreds of billions in our galaxy, so maybe we should expect to be surprised for a long, long time. There is still a lot of weird out there to find.

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