Baked Exoplanet Gets Lab Treatment


Don’t get too excited, an exoplanet hasn’t really been captured from the cosmic wilds. And no, one of NASA’s boffins isn’t really taking a pair of tongs to the upper atmosphere of a strangely tiny “hot-Jupiter” being baked by a Bunsen burner. The doctored photo is actually a fun metaphor for this golden age of exoplanetary science. In particularly, it illustrates what one NASA space telescope is doing to understand the chemistry and dynamics of a particular Jupiter-sized exoplanet located some 385 light-years away.

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Of course, it would be preferential if we could directly sample an exoplanet’s atmosphere in a lab, but as all exoplanets orbit stars many light-years from the nearest Bunsen burner, astronomers need to think up novel techniques by which the atmospheres of exoplanets can be remotely probed. Enter the Spitzer Space Telescope, NASA’s premier infrared observatory, the inadvertent hero of exo-atmospheric science!

Launched in 2003, Spitzer was designed to observe the infrared universe — particularly star-forming molecular clouds and distant galaxies — but in 2005 it became famous for detecting infrared emissions from extra-solar planets, namely HD 209458b and TrES-1. Since then, Spitzer has continued to notch up some impressive exoplanetary discoveries.

“When Spitzer launched in 2003, we had no idea it would prove to be a giant in the field of exoplanet science,” said Michael Werner, Spitzer project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Now, we’re moving farther into the field of comparative planetary science, where we can look at these objects as a class, and not just as individuals.”

In a new study published in the Astrophysical Journal, astronomers have used Spitzer to watch an exoplanet complete a full orbit around its host star.

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Over 6 days, the hot-Jupiter HAT-P-2b passed in front of its star, behind and back in front again. Interestingly, HAT-P-2b’s orbit is highly eccentric, meaning its orbital path takes it only 2.8 million miles from the star’s surface at closest approach and out to 9.3 million miles at its most distant. As a comparison, the solar system’s innermost planet, Mercury, orbits the sun every 88 days and doesn’t come closer than 28 million miles — HAT-P-2b is therefore a roasted planet, where rapid changes in its atmosphere can be expected from extreme heating.

Fortunately, because HAT-P-2b’s orbit is not only compact but also eccentric, astronomers have a wonderful opportunity to see these changes occur over a very short timescale.

“It’s as if nature has given us a perfect lab experiment with this system,” said Heather Knutson, of the California Institute of Technology (Caltech), Pasadena, Calif. (pictured top, poking a simulated exoplanet). “Because the planet’s distance to the sun changes, we can watch how fast it takes to heat up and cool down. It’s as though we’re turning the heat knob up on our planet and watching what happens.”

Spitzer analyzed the infrared light from the exoplanet throughout its orbit. By measuring radiation of different wavelengths, the team of astronomers were able to take a peek at the dynamics of several different layers of HAT-P-2b’s atmosphere, adding depth to the observation. This is the first time a multi-wavelength infrared campaign has been carried out on an exoplanet.

As the exoplanet swings toward the star, the Spitzer data revealed it takes about one day for the exoplanetary atmosphere to heat up before reaching closest approach (periastron). It then takes four to five days to cool down. Because the astronomers had access to temperature-depth measurements, they found that there was a temperature inversion at the hottest point in its orbit — a hotter upper atmospheric layer formed while the lower layers were maintained at a lower temperature.

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The carbon chemistry of the atmosphere was found to be changing in unexpected ways, something the researchers will be trying to understand for some time.

“The hot Jupiters are beasts to handle. They are not fitting neatly into our models and are more diverse than we thought,” Nikole Lewis of the Massachusetts Institute of Technology and lead-author of the paper. “We are just starting to put together the puzzle pieces of what’s happening with these planets, and we still don’t know what the final picture will be.”

“These planets are much hotter and more dynamic than our own Jupiter, which is sluggish by comparison. Strong winds are churning material up from below, and the chemistry is always changing,” she added.

Image: An artistic version of a hot Jupiter inspired by computer simulations has been inserted into a photo showing a Spitzer researcher, Heather Knutson, in a laboratory at the California Institute of Technology in Pasadena, where she works. In reality, Knutson does not work in a lab, nor wear a lab coat and goggles, but scrutinizes telescope data from her office computer. Credit: NASA/JPL-Caltech

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