Planet Hugging a White Dwarf May Be a Survivor of Star's Death Throes
16 September 2020
AUSTIN, Texas — An international team of astronomers has used NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope to discover what may be the first intact planet found closely orbiting a white dwarf, the dense leftover of a sun-like star only 40% larger than Earth. The work, led by Andrew Vanderburg of The University of Texas at Austin, included follow-up observations with the 10-meter Hobby-Eberly Telescope at the university’s McDonald Observatory.
The Jupiter-size object, called WD 1856 b, is about seven times as large as the white dwarf named WD 1856+534. It circles this stellar cinder every 34 hours, more than 60 times as fast as Mercury orbits our sun.
“WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece,” said Vanderburg, who was a NASA Sagan Fellow at UT Austin while completing this work and is now an assistant professor at the University of Wisconsin-Madison. “The white dwarf creation process destroys nearby planets, and anything that later gets too close is usually torn apart by the star’s immense gravity. We still have many questions about how it arrived at its current location without meeting one of those fates.”
A paper about the system, consisting of several co-authors including UT Austin’s Caroline Morley and Andreia Carillo, appears in Nature and is available online.
TESS monitors large swaths of the sky, called sectors, for nearly a month at a time. This long gaze allows the satellite to find exoplanets, or worlds beyond our solar system, by capturing changes in stellar brightness caused when a planet crosses in front of, or transits, its star.
The satellite spotted WD 1856 b about 80 light-years away in the northern constellation Draco. It orbits a cool, quiet white dwarf that is about 11,000 miles (18,000 km) across, may be up to 10 billion years old, and is a distant member of a triple star system.
When a sun-like star runs out of fuel, it swells up to hundreds to thousands of times its original size, forming a cooler red giant star. Eventually, it ejects its outer layers of gas, losing up to 80% of its mass. The remaining hot core becomes a white dwarf. Any nearby objects are typically engulfed and incinerated during this process, which in this system would have included WD 1856 b in its current orbit. The research team estimates the possible planet must have originated at least 50 times farther away from its present location.
“We’ve known for a long time that after white dwarfs are born, distant small objects such as asteroids and comets can scatter inward towards these stars. They’re usually pulled apart by a white dwarf's strong gravity and turn into a debris disk,” said co-author Siyi Xu, an assistant astronomer at the international Gemini Observatory, which is a program of the National Science Foundation’s NOIRLab. “That’s why I was so excited when Andrew told me about this system. We’ve seen that planets could scatter inward, too, but this appears to be the first time we’ve seen a planet that made the whole journey intact.”
The team suggests several scenarios that could have nudged WD 1856 b onto an elliptical path around the white dwarf. This trajectory would have become more circular over time as the star’s gravity stretched the object, creating enormous tides that dissipated its orbital energy.
“The most likely case involves several other Jupiter-size bodies close to WD 1856 b’s original orbit,” said co-author Juliette Becker, a 51 Pegasi b fellow in planetary science at Caltech. “The gravitational influence of objects that big could easily allow for the instability you’d need to knock a planet inward. But at this point, we still have more theories than data points.”
Other possible scenarios involve the gradual gravitational tug of the two other stars in the system, red dwarfs G229-20 A and B, over billions of years and a flyby from a rogue star perturbing the system. Vanderburg’s team thinks these and other explanations are less likely because they require finely tuned conditions to achieve the same effects as the potential giant companion planets.
Jupiter-size objects can occupy a huge range of masses, however, from planets only a few times the mass of Earth to low-mass stars thousands of times Earth’s mass. Others are brown dwarfs, which straddle the line between planet and star. Usually scientists turn to observations to measure an object’s mass, which can hint at its composition and nature. This method works by studying how an orbiting object tugs on its star and alters the color of its light. But in this case, the white dwarf is so old that its light has become both too faint and too featureless for scientists to detect noticeable changes.
Instead, the team observed the system in the infrared using Spitzer, just a few months before the telescope was decommissioned. If WD 1856 b were a brown dwarf or low-mass star, it would emit its own infrared glow. This means Spitzer would record a brighter transit than it would if the object were a planet, which would block rather than emit light. When the researchers compared the Spitzer data with visible light transit observations taken with the Gran Telescopio Canarias in Spain’s Canary Islands, they saw no discernable difference. That, combined with the age of the star and other information about the system, led them to conclude that WD 1856 b is most likely a planet no more than 14 times Jupiter’s size. Future research and observations may be able to confirm this conclusion.
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Rebecca Johnson, Communications Mgr.
The University of Texas at Austin