hubble

All posts tagged hubble

Portrait of a Hot Jupiter

Posted by admin on April 19, 2019
Posted in: BSU Journal Club. Tagged: , , .

Hot Jupiters are gas giant planets like Jupiter but orbiting so close to their host stars that they can be as hot as small stars. In fact, some suffer such strong irradiation that their atmospheres are being blasted off into space. These objects were among the first exoplanets detected, and their origins and fates remain unclear.

Being so hot, their atmospheres are also unlike any planet’s in our solar system, but we can be certain that, with temperatures hot enough to vaporize rock, the weather on these planets is highly dynamic. The video below shows what happens to one such planet as it gets blast-roasted by its host star – a giant thermal wave screams around the planet.

An animation showing meteorological chaos on HD80606b.

Weather forecasts on hot Jupiters would to have keep track of super-sonic winds, ruby rain, and warm fronts heated by magnetic fields. And a recent study using observations from the Hubble Space Telescope shows that these dynamic atmospheres remain mysterious.

Led by Jacob Arcangeli of the University of Amsterdam, the study used the Wide-Field Camera onboard Hubble to watch WASP-18 as the hot Jupiter circled its star.

As the planet swings revolves around its star, we see first the cool nightside of the planet and then the blistering dayside. By measuring the diurnal cycle of infrared light emitted from the two sides of the planet, called the phase curve, Arcangeli and colleagues were able to measure their respective temperatures and learn something about the planet’s weather.

The phase curve of WASP-18b measured by Hubble (shown in blue), along with model fits.

The figure above shows WASP-18b’s phase curve as blue points and a model fit to the data as a black line. By applying computer simulations for the planet’s weather, Arcangeli and colleagues estimated what they expected the phase curve to look like various assumptions about the planet’s composition and atmospheric dynamics.

A map of atmospheric temperatures (in degrees Kelvin) in WASP-18b’s atmosphere.

Using their models, they were able to draw a rough map of temperatures in WASP-18b’s atmosphere (shown above), like a weather map for the Earth. Like most hot Jupiters observed show far, the hottest place in WASP-18b’s atmosphere lies a little east of the point that receives the most sunlight, the substellar point. That’s because high windspeeds blow the strongly heated gas away toward the east, a little like the jet stream on Earth.

However, the winds inferred from the Hubble observations weren’t as strong as expected from the model, suggesting there is some form of drag or friction in WASP-18b’s atmosphere.

The likely culprit: the planet’s very own magnetic field. Gas in WASP-18b’s atmosphere is actually so hot, it can become somewhat plasma-fied, and plasma, consisting of hot charged particles, can interact with the hot Jupiter’s magnetic field.

This is totally unlike planets in our own solar system, where the atmospheres are comparatively cool and none of the gas has turned into plasma.

The upshot of this result is that we may be able to use observations of the meteorology on distant worlds to learn something about the planets’ magnetic fields, which originate deep inside the planet. So their weather may allow us to plumb the depths of these distant worlds.

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.

Observing the Atmospheres of TRAPPIST-1 Planets

Posted by admin on February 16, 2018
Posted in: Journal Club. Tagged: , , , .

Comparing the TRAPPIST-1 system to the solar system. From http://www.spitzer.caltech.edu/images/6294-ssc2017-01h-The-TRAPPIST-1-Habitable-Zone.

In astronomy, when you look for evidence supporting a hypothesis and don’t find it, that’s called a “null result“. The null result is usually not all that exciting, but last week, an attempt to detect the atmospheres of potentially habitable exoplanets came up null, and that, as it turns out, may mean the planets are habitable.

In a recent study published last week in Nature Astronomy, Dr. Julien de Wit of MIT and colleagues observed the transits of four of the planets in the TRAPPIST-1 system. The discovery of this system was announced last year and generated a lot of interest — it comprises seven Earth-sized planets orbiting a red-dwarf star, and at least four of the planets orbit in the star’s habitable zone. So astronomers are scrambling to determine the climatic conditions on these planets and find out whether they host life.

Key to those conditions is the composition of the planets’ atmospheres, and the best way to probe atmospheres lightyears distant is to detect the colors of the planets’ shadows as they pass in front of their host stars, i.e. as they transit. When light from the star passes through a planet’s atmosphere, the cool gases can imprint spectral signatures, which we can then detect using a very sensitive telescope — de Wit used the Hubble Space Telescope.

Now, even though the TRAPPIST planets are Earth-sized, that doesn’t mean their atmospheres are Earth-like — astronomers have founds lots of weird planets in the last several years. The atmospheres could be hydrogen-rich like Jupiter, hydrocarbon-rich like Neptune, or rich in nitrogen and oxygen like Earth.

In principal, each kind of atmosphere would give a distinct spectrum, but in practice, atmospheres rich in hydrocarbons or nitrogen, potentially good atmospheres for life, are difficult to detect because they are weighty and drape over the planet like a heavy blanket. By contrast, a hydrogen-rich atmosphere, although probably not great for life, can be light and fluffy, relatively easy to detect.

When de Wit and colleagues analyzed transit data they collected from Hubble showing transits for planets TRAPPIST-1 d, e and f, they found no atmospheric signals down to their detection limits. The spectra below show this lack of atmospheric coloration and what they would have detected if the atmosphere was hydrogen-rich.

Now, of course, this non-detection does NOT mean the planets are habitable or even Earth-like. As the figure shows, their atmospheres could still be radically different from the Earth’s (drenched in water vapor or carbon dioxide-rich like Venus), but it rules out the possibility that they are Jupiter-like — potentially good news for life there.

Very likely, when the James Webb Space Telescope finally launches next year, the TRAPPIST-1 system will be one of its first targets. JWST’s vastly improved sensitivity will help reveal not just a potent null result for the TRAPPIST-1 system but may also reveal the glimmer of distant Earth-like worlds.

Spectra for TRAPPIST-1 d, e, f, and g. From de Wit et al. (2018) – https://www.nature.com/articles/s41550-017-0374-z.