Hubble Observes Alien Planet’s Stratosphere for the First Time

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The Hubble Space Telescope may be old, but it’s often still the best way for astronomers to observe distant objects. For example, the massive exoplanet WASP-121b orbiting a star roughly 900 light years from Earth. Observations of this so-called “hot Jupiter” show that it has a warm stratosphere, a common trait of planets in our own solar system. That’s a major step toward understanding the atmosphere of exoplanets.

A stratosphere is a layer in the atmosphere of a planet in which the temperature increases along with the altitude. Planets in our solar system have them, but we could only assume that exoplanets were the same. On Earth, the stratosphere is heated by ozone that traps ultraviolet radiation. On Jupiter and Titan, it’s methane that causes stratospheric heating.

Scientists used data from Hubble to analyze WASP-121b, which is 1.2 times as massive and 1.9 times as large as Jupiter. It has an orbital period of just 1.3 days, whereas Jupiter orbits the sun once every 12 years. Its close proximity to the host star is why it’s called a hot Jupiter — temperatures in the planet’s atmosphere are as high as 4,600 degrees Fahrenheit (2,500 Celsius).

Being 900 light years away, there aren’t many telescopes that can get any data from the planet, which is blotted out by the star. Because WASP-121b transits in front of its star from our perspective, Hubble was able to observe how different molecules in its atmosphere reacted to light — this is known as spectroscopy. Specifically, the spectrum of light is affected in predictable ways when passing through water vapor at varying temperatures. The emissions from water vapor in WASP-121b’s atmosphere indicate the temperature is rising with the altitude. Thus, WASP-121b most likely has a stratosphere.

Now that we have good evidence stratospheres exist in exoplanet atmospheres, the planetary models based on our solar system should apply well to those too far away to inspect in detail. We lack the tools to get a good look at WASP-121b. We don’t have any hot Jupiters to study closely, but the regular Jupiter is comparatively nearby and might have a similar atmospheric composition.

Astronomers plan to do more observations of WASP-121b with Hubble in the coming months, but the real fun begins when the James Webb Space Telescope comes online. It’s currently scheduled for a 2018 launch. The more powerful instruments on this telescope will be more able to tease out data from exoplanets than the quarter-century old Hubble.

Now read: The 25 Best Hubble Space Telescope Images

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