Taylor Bell, along with his team at McGill University and the University of Exeter used the Space Telescope Imaging Spectrograph on the NASA/ESA Hubble Space Telescope to measure the albedo of the exoplanet WASP-12b’s.
In other words, they measured how much light is reflected off its surface. What they found was that WASP-12b has an extremely low albedo, making it appear nearly pitch black.
“We did not expect to find such a dark exoplanet,” Bell said, according to NASA. “The low albedo shows we still have a lot to learn about WASP-12b and other similar exoplanets,” he explained, according to ScienceDaily.
WASP-12b is in the constellation Auriga, orbiting around a star that is 1400 light years away from Earth. It is a part of a category of planets called “hot Jupiters.”
These are large gaseous planets even bigger than our own Jupiter that orbit so close to their sun that their surfaces are heated to thousands of degrees Fahrenheit.
Their temperatures can be so hot that stellar models must be used to predict their atmospheres.
Astronomers know that these gigantic planets could not have been born in their current orbitals so near to their sun. So, scientists have theorized that they originated further out in their solar system and gravitated inwards, unlike our own Jupiter which seems not to have strayed far from its origin.
However, astronomers have yet to pinpoint what causes this planetary migration. Some theories assert that during the early formation of a solar system, these large planets slid smoothly inward through the planet-forming disks, or that, later in their life, these planets adopted an eccentric orbit (much like a comet) and eventually their orbits settled into the circular pattern that astronomers observe today. Current research into exoplanet HD80606b, a hot Jupiter caught in this eccentric orbit, shows that the later theory may be a process that takes billions of years, making it implausible.
Originally, these hot Jupiters were thought to be uncommon, since they don’t exist in Earth’s solar system. In recent years more and more of these gigantic planets have been found orbiting our stellar neighbors, making it possible that Earth’s solar system is the oddball.
The hot Jupiter WASP-12b, the subject of Bell’s study, orbits two million miles from its sun, giving it a year equivalent to one Earth day. It is tidally locked, meaning that one side is always facing the sun.
The gravity of its sun has pulled the planet into an oblong-egg shape, and the night side of WASP-12b is 2000 degrees Fahrenheit cooler than the side facing its sun.
Using the Hubble Space Telescope, the team was able to accurately measure WASP-12b’s albedo, or the proportion of light that is reflected off of a planet’s surface.
To do so they measured the visible starlight from WASP-12b’s sun during an eclipse. Usually the planet reflects starlight, increasing the actual output of starlight, but when WASP-12b passed behind the sun, the added reflection was gone and the total starlight diminished. This change in light can be used to measure WASP-12b’s albedo, which had a maximum of 0.064.
“This is an extremely low value, making the planet darker than fresh asphalt,” Bell said, according to ScienceDaily.
This is two times less reflective than the moon, which has an albedo of 0.12.
Such a low albedo seemed confounding, since it didn’t match the two previous theories for WASP-12b’s atmosphere.
“Most hot Jupiters reflect about 40 percent starlight,” Bell said according to NASA.
Bell also explained how some other hot Jupiters can have low albedos.
“It is suggested that things like clouds and alkali metals are the reason for the absorption of light,” he said.
However, WASP-12b is so incredibly hot that clouds can’t form and alkali metals are broken down. It is the temperature of WASP-12b that may lend to its high absorption of light.
In WASP-12b’s atmosphere, hydrogen molecules are broken down into atomic hydrogen, which soak up light from its star and converts it into energy. Bell explained what this discovery meant to the overall study of Jupiter-liek planets.
“This new Hubble research further demonstrates the vast diversity among the strange population of hot Jupiters [in the universe],” he said.
