Disintegrating Planets Could Be the Key to Discovering What Worlds Are Made Of
Exoplanets that have been ripped apart by their host stars could provide an opportunity to measure the minerals of the galaxy
The exoplanet Kepler-1520b is so close to its host star that it completes an orbit in just over half a day. At this close proximity, Kepler-1520b is tidally locked in a gravitational stability, keeping one half of the planet facing the star and the other half facing away at all times. Unfortunately for Kepler-1520b, this arrangement turns the star-facing side of the planet into a churning mass of molten rock and magma seas, slowly boiling off into space.
Even though Kepler-1520b is not long for this galaxy, astronomers are eager to learn more about the disintegrating world, positioned about 2,000 light-years from Earth. The planets' comet-like tail of dust and debris could provide insight into the fundamental formation process of all planets in the galaxy. New telescopes, such as NASA's James Webb Space Telescope scheduled to launch in 2021, may be able to probe the cloud behind Kepler-1520b and two other slowly disintegrating worlds.
"The composition in an exoplanet system could be substantially different from the solar system," says Eva Bodman, an exoplanet researcher at Arizona State University. As more and more exoplanets are discovered, astronomers are struck by how unique our solar system looks from other planets orbiting other stars. Bodman set out to determine if it was possible to measure the composition of a small, rocky, disintegrating exoplanet by studying the debris traveling in its wake. But there was a problem.
Spotting the fingerprint of rocky elements requires studying the worlds in infrared. Ground-based telescopes aren't sensitive enough to spot them, leaving only NASA's soon-retiring Spitzer Space Telescope and SOFIA, a telescope carried above the atmosphere on board a Boeing 747. Neither instrument has the range to look for the rocky material, Bodman says. But James Webb, designed to study exoplanets in infrared as well as ancient galaxies and the most distant objects of the universe, should be able to peer through the clouds of debris and identify some of their ingredients.
"Webb would be able to measure the relative abundances of different minerals," Bodman says. "From that, we can infer the geochemistry of the interior of these planets was before they started disintegrating." Bodman and her team’s findings on the feasibility of studying disintegrating exoplanets were published in the Astronomical Journal late last year.
In 2012, scientists reviewing data from NASA's Kepler space telescope found signs of a world being slowly shredded by heat and pressure, Kepler-1520b. Two more shredded planets were found in the following years among the thousands of exoplanets discovered by Kepler and its extended mission, K2. Circling their stars in just a handful of hours, these rocky bodies boast temperatures as high as 4,200 degrees Celsius (7,640 degrees Fahrenheit) on the superheated regions facing the stars.
The extreme temperatures drive the planet's dissolution. "The atmosphere is just rock vapor," Bodman says. "It's the sheer heat of the planet that's pushing off this rock vapor atmosphere."
Radiation produced by the stars pushes against the planet’s vaporized atmospheres, creating a cloudy tail. Although Kepler wasn't able to directly measure how large the shrouded planets were, simulations suggest that they are between the size of the moon and Mars. Any more compact, and the disintegration process shuts down.
These objects were not always so small and shriveled, however. Kepler-1520b and the two other objects like it are thought to have formed as gas giants, after which they migrated in toward their host stars and were stripped all the way down to the rocky core.
In recent years, exoplanet scientists have made great strides studying the atmospheres of large, gaseous planets orbiting other stars. Most of that material is rich in hydrogen and helium and can be identified using NASA's Hubble Space Telescope. But the rocky materials fall on a different part of the spectrum, "in wavelengths that Hubble can't currently reach," says Knicole Colon, a research astrophysicist at NASA's Goddard Space Flight Center in Maryland who has studied the disintegrating planet K2-22. "With James Webb, we'd be able to go out to those wavelengths.”
Using Webb to hunt for materials such as iron, carbon and quartz, astronomers would gain a better understanding about what's going on inside distant worlds. "If we were able to detect any of these features, we could say with some certainty what these rocky bodies are made off," Colon says. "That could definitely be very informative for understanding rocky exoplanets in general."
Planets form from the cloud of dust and gas leftover after the birth of a star. Scientists think the worlds of the solar system were created by a process known as pebble accretion, in which small bits of dust and gas come together to make larger and larger objects. Eventually, the cores of the gas giants grow massive enough to attract leftover gas, forming their thick atmospheres. But the exact steps remain difficult to pin down.
The interiors of planets around other stars would vary depending on the elements found in that particular environment. Sorting through these differences could help researchers better understand those tantalizing first steps of planet formation.
"There's no reason that the solar system should be different from exoplanets, and vice versa," Colon says. "We're all planets, so we all formed in possibly similar ways. Understanding these planets is another step in the process to the bigger picture."
But even with similar formation processes, Bodman suspects that planets around other stars might not look so familiar. "The composition in an exoplanet system could be substantially different from the solar system," she says.
Although Webb will only be able to tease out information about exoplanet composition, advanced instruments may one day allow disintegrating planets to reveal even more about themselves. As the planets erode away, astronomers could get an unprecedented look at their interiors, possibly down to the core. "In theory, we could know more about these exoplanets than even about the Earth, and definitely more than the other planets in the solar system," Bodman says.
Unlike stars, which can shine for tens of billions of years, shredded worlds only stick around for a relatively short time. Simulations suggest that planets like K2-22 only have about 10 million years before they are completely destroyed. And because all three worlds orbit stars that are billions of years old, they probably haven't been in their current positions for very long.
Bodman and Colon both think the doomed planets probably formed far out in their system and then migrated inward over time. Interactions with other planets could have hurled them on their fateful trajectories, although all three of these disintegrating planets are the only known satellites of their host stars. Bodman says it is likely the worlds have only recently begun a close orbit of their stars, but how they got there remains an open question.
The short lifetime of a disintegrating planet—only a blip in the longer life of a star—is probably why so few of these worlds have been found. "They're definitely rare," Bodman says.
Both women agree that there is a good chance that another one or two disintegrating exoplanets are contained in the Kepler data, especially the most recent results from K2. And the recently launched Transiting Exoplanet Survey Satellite (TESS), which has already found hundreds of new planets, will produce even more.
"I think it'll take some time to sift through everything, but I'm hoping we find more," Colon says.