Astronomers using the Gran Telescopio Canarias have spotted a low-mass planet-like object in a disk of gas and debris around SDSS J122859.93+104032.9, a white dwarf star located approximately 410 light-years away. The researchers speculate that it may be the leftover core of a planet whose outer layers have been removed.
A planet-like object orbits SDSS J122859.93+104032.9, leaving a tail of gas in its wake. Image credit: Mark Garlick / University of Warwick.
White dwarfs are the remains of Sun-like stars that have burnt all their fuel and shed their outer layers, leaving behind a dense core which slowly cools over time.
SDSS J122859.93+104032.9 (J1228 for short) has shrunk so dramatically that the newly-discovered object orbits within its star’s original radius.
“The original star would have originally been about two solar masses, but now the white dwarf is only 70% of the mass of our Sun,” said Dr. Christopher Manser, an astronomer at the University of Warwick, UK.
“J1228 is also very small — roughly the size of the Earth — and this makes the star, and in general all white dwarfs, extremely dense.”
“Its gravity is so strong — about 100,000 times that of the Earth’s — that a typical asteroid will be ripped apart by gravitational forces if it passes too close to the white dwarf.”
Dr. Manser and colleagues estimate that the planet-like object has to be at least one km in size, but could be as large as a few hundred km in diameter, comparable to the largest asteroids known in our Solar System.
“Our discovery is only the second solid planetesimal found in a tight orbit around a white dwarf, with the previous one found because debris passing in front of the star blocked some of its light — that is the transit method widely used to discover exoplanets around Sun-like stars,” Dr. Manser said.
“To find such transits, the geometry under which we view them has to be very finely tuned, which means that each system observed for several hours mostly leads to nothing.”
“The spectroscopic method we developed in this research can detect close-in planetesimals without the need for a specific alignment.”
“We already know of several other systems with debris discs very similar to J1228, which we will study next.”
“We are confident that we will discover additional planetesimals orbiting white dwarfs, which will then allow us to learn more about their general properties.”
The discovery is reported in the April 5 issue of the journal Science.
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Christopher J. Manser et al. 2019. A planetesimal orbiting within the debris disc around a white dwarf star. Science 364 (6435); doi: 10.1126/science.aat5330
