Artist's impression of WASP-12b. (NASA, ESA, and G. Bacon/STScI)

One of The Blackest Planets in The Galaxy Is Headed For a Fiery Death

8 DECEMBER 2020

WASP-12b is one of the more interesting exoplanets we know of. Orbiting a yellow dwarf star a little bigger than the Sun 1,410 light-years away, the ultra-black planet is what's known as a "hot Jupiter" - a gas giant exoplanet with similar mass and size to Jupiter, but so close to the star that it's scorching hot.

 

WASP-12b has never exactly been in the most secure position. With an orbital period of just over a day, the gas giant exoplanet is so close to its star that a constant stream of material is being siphoned away from its atmosphere.

But its death won't necessarily be by slow stellar slurping. Careful observations have found it's also on a noticeably decaying orbit. And, according to new research, that orbit is decaying a bit faster than we initially thought.

Rather than the 3.25 million years initially estimated, WASP-12b will meet its fiery end in just 2.9 million years.

According to current models of planet formation, technically hot Jupiters shouldn't exist. A gas giant can't form that close to a star because the gravity, radiation, and intense stellar winds ought to keep the gas from clumping together. But they do exist - several hundred have been identified in the exoplanet data.

However they form, hot Jupiters that are particularly close to their star are some of the most studied exoplanets out there. This is because they can tell us a lot about the tidal interactions between a planet and a star.

 

WASP-12b is among the closest hot Jupiters to its star. And it's been an excellent example for studying tidal interactions.

It was discovered in 2008, which means astronomers have been able to collect a relatively long-term dataset; and its short orbit means that we can observe a lot of transits. That's when the exoplanet passes between us and the star, causing the latter's light to ever so slightly dim.

It was in 2017 that astronomers noticed something strange about WASP-12b's transits. They were occurring just a fraction of a second off when they should have been, based on previous measurements of the orbital period.

That slight timing variation could have been the result of the exoplanet's orbit changing direction, so a team of astronomers led by Samuel Yee of Princeton University decided to closely examine not just the transits, but the occultations, when the exoplanet passes behind the star. If WASP-12b was changing direction, the occultations should be slightly delayed.

A transit causes a faint dimming of the star's light; an occultation causes an even fainter dimming. This is because the exoplanet, reflecting the star's heat and light, adds to the system's overall brightness when it's not behind the star.

 

WASP-12b is very dark, optically; it absorbs 94 percent of all light that shines on it, making it blacker than asphalt.

Astronomers believe that this is because the exoplanet is so hot; at 2,600 degrees Celsius (4,700 degrees Fahrenheit) on its day side, hydrogen molecules are broken down into atomic hydrogen, causing its atmosphere to behave more like a low-mass star. But because it's so hot, it glows in infrared.

Yee's team used the Spitzer Space Telescope to try to observe occultations. Although they observed the star, WASP 12, for 16 orbital periods, they only managed to find four faint occultations in the data. It was enough, though.

These occultations could be matched to transits… and the researchers found that the occultations were occurring more quickly - consistent with an orbital decay of 29 milliseconds per year. At that rate, the planet's lifespan was, the astronomers calculated, around 3.25 million years.

Now, a new team of researchers led by Jake Turner of Cornell University has looked for signs of orbital decay in a different dataset - observations taken by NASA's planet-hunting telescope TESS, specifically designed to observe transits and occultations.

TESS studied the region of the sky that included WASP-12 from 24 December 2019 to 20 January 2020. In this data, the team found 21 transits. The occultations were too shallow to be detected individually, but the team was able to model them to find a best-fit for the TESS data.

These transit and occultation times were combined with the earlier data for a timing analysis. And Turner and his team were able to confirm that WASP-12b's orbit is indeed decaying. But it's doing so a little faster than we thought - at a rate of 32.53 milliseconds per year, for a total lifespan of 2.9 million years.

That sounds like a long time, but on cosmic timescales, it's practically an eyeblink. And it has dramatically shortened the exoplanet's lifespan from the estimated 10 million years it would take for the planet to die from atmospheric stripping.

But, although it doesn't have long to live, studying WASP-12b has the potential to teach us a lot. And while it's the only exoplanet for which we have robust evidence of orbital decay, there are other hot Jupiter exoplanets that are expected to exhibit similar rates of orbital decay.

"Hence, additional data could reveal whether [these exoplanets] indeed exhibit hitherto undetected tidal decay or whether the theoretical predictions need to be improved," Turner and his team wrote.

"Timing observations of additional systems are warranted because they help us understand the formation, evolution and ultimate fate of hot Jupiters."

The team's research has been accepted into The Astronomical Journal and is available on arXiv.