transiting planets

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Helium in the eroding atmosphere of an exoplanet

Posted by admin on February 22, 2019
Posted in: BSU Journal Club. Tagged: , , , .

Artist’s conception of a hot Jupiter shedding mass.

The very first exoplanet discovered around a Sun-like star, 51 Peg b, was a shocker – it’s a giant planet like Jupiter made mostly of hydrogen and helium but 100 times closer to its sun than Jupiter is to ours and whizzes around its orbit every 4 days.

Indeed, when its discoverers Michel Mayor and Didier Queloz first spotted the telltale spectral wobble of a planet in a 4-day orbit, they didn’t believe their discovery. At the time, everyone knew (or thought they knew) that planets like Jupiter could only form very far away from their host star.

Worse, so close to its star, 51 Peg b’s was being super-heated, and Mayor and Queloz worried that such a hot gas giant might quickly lose its hot, bloated atmosphere. And in their discovery paper, they suggested that the giant planet we see today as 51 Peg b might have started out as a brown dwarfthat shed trillions and trillions of lbs.

Later studies showed those early concerns about atmospheric blow-off were overblown and planets as massive as 51 Peg b, even if they are as scorched, probably can’t lose more than a fraction of their original mass. Since then, hot Jupiters like 51 Peg b, while cosmically rare, have become a fairly common type of exoplanet discovery.

But that doesn’t mean these planets aren’t losing a lot of mass, and a recent study from David Sing and colleagues looks at one of the mass-losing-est planets we know of, WASP-107b

Artist’s conception of WASP-107b transiting its host star.
From https://en.wikipedia.org/wiki/WASP-107b.

Sing and colleagues collected transit observations in infrared wavelengths of the WASP-107 system using the venerable Hubble Space Telescope. By looking in the infrared, they could search for the spectral signals of different gases in WASP-107b’s atmosphere.

WASP-107b is an especially good target for atmospheric characterization because its host star is very bright (compared to other planet hosts) and the planet itself is very low density – it has a mass a tenth that of Jupiter’s but a radius almost as big, giving the planet a density comparable to wind-packed snow.

With such a low density, WASP-107b’s atmosphere is puffy and distended, which means that its atmospheric gases can easily imprint their spectral signatures on the light observed by Hubble, making them easy to detect.

And for the first time in any exoplanet, Sing and colleagues saw signs of helium gas in WASP-107b’s atmospheric spectrum. In fact, the helium signal they saw was so whopping big that it suggests WASP-107b’s atmosphere is actively escaping, at a rate of about 10,000 tons per second.

Escape of WASP-107b’s atmosphere. The planet is the small grey circle near bottom, the star is the yellow circle, and the escaping atmosphere is shown in blue. The black line is the planet’s orbit. From Sing et al. (2018).

Even with such a high escape rate, WASP-107b won’t fall apart anytime soon – Sing and colleagues estimate it would only lose about 4% of its mass in a billion years.

But as we continue to find more exoplanets, we should probably expect to find more even closer to their host stars with even puffier atmospheres, perhaps some on the verge of being gravitationally ripped apart. So as with 51 Peg b’s discovery, exoplanets are likely to keep challenging our preconceived notions about where planets can and cannot be.

Hot Jupiters and Zeno’s Tortoise

Posted by admin on February 23, 2018
Posted in: Journal Club. Tagged: , , , .

With the very first discovery of an exoplanet around a Sun-like star (51 Peg b), astronomers were introduced to hot Jupiters. These totally unexpected planets resemble Jupiter in mass, composition, and size, but they have orbits that nearly skim the surfaces of their host stars. Some of them are even losing their atmospheres under the apocalyptic glare of their host stars.

How their lives began remains a mystery, but we have a pretty good idea of how their lives will end – they will be engulfed or torn apart by their host stars. That’s because hot Jupiters are big and close enough that they can actually raise a tidal bulge on the stars (and we can actually see the bulge in a handful of cases).

This tidal interaction can cause the planets to spiral downward toward the stars, and at the same time, it causes the spins to spin faster until the planet is destroyed by the star. The same tidal effect, just in reverse, is driving the Moon away from the Earth, while slowing down the Earth’s spin. But here’s the key: we don’t know how quickly the planets are spiraling in.

Tidal decay of planetary orbital period over billions of years (Gyrs). From Penev et al. (2018 – https://arxiv.org/abs/1802.05269).

Enter Prof. Kaloyan Penev of UT Dallas Physics Dept. On Valentine’s Day last week, he and his colleagues published an academic love note exploring planetary tidal decay. To do this, they modeled the evolution of planetary orbits and stellar spins under the influence of tides. The tracks in the figure at left show how a planet’s orbital period (or distance from its star) might shrink over billions of years, thanks to tides. The clump of spaghetti noodles in the figure shows that evolution for a range of assumptions about the rate of decay.

By comparing the stellar spin rate and planetary orbit predicted by their model to those we actually observe for each system, Penev and colleagues showed that the tidal decay rates might actually slow down as the planets approach their stars. So perhaps instead of an reckless death dive into the star over a few million years, the planets make like Zeno’s tortoise and tiptoe closer and closer without plunging in.

Upcoming surveys such as the TESS mission and the Large Synoptic Survey Telescope may soon allow us to test whether planets do or do not plunge into their stars. Theoretically, we expect stars that eat their planetary children dramatically brighten up by a factor of 10,000 over a few days – faster than a supernova brightens but nowhere near as bright. These surveys might able to see stars engaged in this act of cosmic infanticide.