WASP-12 b is in trouble. A giant ball of mostly hydrogen, the planet circles its star once every 25 hours. The resulting intense stellar irradiation drives super-sonic storms of plasma around the world, and the atmosphere has so much thermal energy, in fact, that some of it is escaping into space. But it gets worse. WASP-12 b is steadily tumbling toward its host star, and astronomers expect that, within a few million years, the star will eat the planet.
WASP-12 b is one of a few hundred hot Jupiters, gas giants very close to their stars, and so far, it’s the only one we have confirmed in a death spiral. Many other hot Jupiters probably are probably also condemned, but how many more can we find perched on the edge of destruction? And, come to think of it, how did the planets find themselves in such precarious positions in the first place? To answer these questions, astronomers need to understand how many hot Jupiters there are out there and how many more are left to be found.
Party Like It’s 1995
In 1995, Michael Mayor and Didier Queloz announced the discovery of the very first extrasolar planet around a Sun-like star, 51 Pegasi b (it was not, however, the first exoplanet discovered). Although this discovery revolutionized astronomy (indeed, it won Mayor and Queloz the Nobel Prize in 2019), it was nothing like what was expected. In his book, New Worlds in the Cosmos, Mayor describes their initial reaction: “We couldn’t keep this mysterious object out of our minds. Its strange properties [were] so unexpected, so clearly absurd…”.
All these first exoplanets discovered around Sun-like stars, 51 Pegasi b included, were hot Jupiters, but these weren’t the first type of planet found because they are the most common type. Like the loudest guest at a house party, they are simply the easiest to notice. With their hulking sizes and tiny orbits, hot Jupiters yank their host stars around, giving rise to a large Doppler signal that astronomers can more easily detect in the stars’ spectra. Hot Jupiters that transit their stars also cast very deep shadows that can even be detected by amateur-class telescopes.
Because of these detection biases, the number of hot Jupiters we’ve detected is not the same as the actual number of hot Jupiters in our galaxy. It’s not even the same as the number of hot Jupiters we *might* be able to detect but just haven’t yet. So what weird and wild worlds await wevelation?
All the Worlds We Cannot See
That’s the question Samuel Yee, an astronomy graduate student at Princeton, recently set out to answer. In a recent paper, he and his colleagues look at all 40 hot Jupiters seen transiting by NASA’s Kepler Mission. With these planents, Yee and colleagues tried to estimate how many other transiting hot Jupiters might still be detectable but not yet detected. So how did they use planets they could see to count up planets they didn’t?
Imagine you were the stocking manager for the Boise State bookstore, and you wanted to know how many orange and blue Boise State t-shirts to buy for the fans attending an upcoming football game. You could, for example, look at a photo of a recent game and count up the proportion of blue and orange t-shirts.
To save time, you decide to count only a few stadium sections, assuming that those sections are representative of all sections. Unfortunately, the photo was taken at night, so some of the sections are not very well lit. You guess that you can accurately count only about half the people those sections. So you do your count and then inflate the number of counted orange and blue t-shirts both to account both for the darkening (multiplying by two) and the total number of sections (dividing by ratio of counted to total sections).
This is the kind of statistical reasoning that astronomers do all the time and that Samuel Yee and colleagues did to extrapolate from Kepler’s transiting hot Jupiters. For instance, since it was harder for Kepler to detect transits for dimmer stars, they scale up the number of transiting hot Jupiters around dimmer stars.
After correcting for all the detection biases they could, Yee and colleagues suggest that, although Kepler was wildly successful finding planets, there are still a lot of hot Jupiters out there, awaiting discovery. Yee estimates that more than 500 additional hot Jupiters may be found by Kepler’s successor, the ongoing TESS mission.
Coming Home Again
That’s a lot of t-shirts to count. So what might these new planets tell us? Well, astronomers still don’t understand where hot Jupiters come from. Some astronomers have suggested they form much farther from their host stars and only reach their perilous orbits after violent orbital perturbations. Other theories posit that the planets were born hot.
Figuring out which, if either, of these theories explains their origins is key for understanding the early histories of all planetary systems since their enormous masses mean Jupiter-sized planets are the bullies of their planetary systems. Indeed, previous studies suggest that hot Jupiters are lonely, with few to no other planets in their systems. This result probably means that the process that makes hot Jupiters destabilizes any other planets in the system, throw them into their stars or ejecting them from their systems.
The fact that our solar system has so many small planets orbiting alongside two Jupiter-sized planets may mean that we never had a hot Jupiter. On the other hand, the Sun shows an unusual chemical composition compared to other similar stars, consistent with the possibility that our system long ago hosted a hot Jupiter that was later swallowed by the Sun.
So as exotic as hot Jupiters may seem, they may actually be more familiar than we ever expected.