The violent events that led to the death of a star would likely drive planets away. The newly discovered Jupiter-sized object may have arrived long after the star died.
An international team of astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope have reported what may be the first intact planet found close to a white dwarf, the dense remnant of a sun-like star. only 40% larger than Earth.
The Jupiter-sized object, called WD 1856 b, is about seven times larger than the white dwarf named WD 1856 + 534. It orbits this star every 34 hours, more than 60 times faster than mercury orbits our sun.
How could a giant planet have survived the violent process that turned its parent star into a white dwarf? Astronomers have a few ideas after discovering the Jupiter object WD 1856 f. Credit: NASA / JPL-Caltech / NASA’s Goddard Space Flight Center
“WD 1856 b somehow came very close to its white dwarf and managed to stay in one piece,” said Andrew Vanderburg, an assistant professor of astronomy at the University of Wisconsin-Madison. “The white dwarf creation process is destroying nearby planets, and anything that comes too close later is usually torn up by the star’s enormous gravity. We still have many questions about how WD 1856 b arrived at its current location without encountering one of These fates. “
A paper on the system, led by Vanderburg and including several NASA co-authors, appears in the Sept. 17 issue of Nature and is now available online.
TESS monitors large shards of sky, called sectors, for almost a month at a time. This long gaze allows the satellite to find exoplanets or worlds beyond our solar system by capturing changes in star brightness caused when a planet crosses in front of or passes through 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 18,000 kilometers above, can 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 springs out its outer layers of gas and loses up to 80% of its mass. The remaining hot core becomes a white dwarf. Any nearby objects are typically engulfed and burned during this process, which in this system would have included WD 1856 bi its current orbit. Vanderburg and his colleagues estimate that the possible planet must originate at least 50 times further away from its current location.
“We have long known that after white dwarfs are born, small objects such as asteroids and comets can disperse inward toward these stars. They are usually pulled apart by a white dwarf’s strong gravity and become a dirt disk,” the co-author said. Siyi Xu, an assistant astronomer at the International Gemini Observatory in Hilo, Hawaii, 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 hints that planets could also spread inward, but this seems to be the first time we’ve seen a planet that made the entire journey intact. . “
The team suggests several scenarios that could have pushed WD 1856 b on an elliptical path around the white dwarf. This orbit would have become more circular over time as the star’s gravity stretched the object, creating huge tides that scattered its orbital energy.
“The most likely case involves several other Jupiter bodies close to the original orbit of WD 1856 b,” said co-author Juliette Becker, a 51 Pegasi b fellow in planetary science at Caltech in Pasadena. “The gravitational influence of objects that are large could easily enable the instability you 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 pull of the other two stars in the system, red dwarfs G229-20 A and B, over billions of years and a flyby from a rogue star that disrupts the system. Vanderburg’s team believes that these and other explanations are less likely because they require fine-tuned conditions to achieve the same effects as the potential giant companion planets.
Jupiter-sized objects can occupy a large variety of masses, though only from planets a few times more massive than Earth to stars with low masses thousands of times the mass of Earth. Others are brown dwarfs that extend across the line between planet and star. Usually, scientists turn to radial velocity observations to measure the mass of an object, which may indicate its composition and nature. This method works by studying how an orbiting object draws in its star and changes the color of light. But in this case, the white dwarf is so old that its light has become both too dim and too functional for scientists to detect noticeable changes.
Instead, the team observed the infrared system using Spitzer, just a few months before the telescope was introduced. If WD 1856 b were a brown dwarf or low-mass star, it would emit its own infrared glow. This means that Spitzer would record a brighter transit than it would if the object were a planet blocking rather than emitting light. When the researchers compared the Spitzer data with visible light observations taken with the Gran Telescopio Canarias in the Canary Islands of Spain, they saw no noticeable difference. This, 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 the size of Jupiter. Future research and observations may confirm this conclusion.
Finding a possible world close to a white dwarf made co-author Lisa Kaltenegger, Vanderburg, and others consider the consequences of studying atmospheres in small rocky worlds in similar situations. For example, suppose a ground-sized planet was located in the area of orbital distances around WD 1856, where water could exist on its surface. Using simulated observations, scientists show that NASA’s upcoming James Webb Space Telescope was able to detect water and carbon dioxide in the hypothetical world by observing only five passages.
The results of these calculations, led by Kaltenegger and Ryan MacDonald, both at Cornell University in Ithaca, New York, have been published in The Astrophysical Journal Letters and are available online.
“Even more impressively, Webb was able to detect gas combinations that potentially indicate biological activity in such a world in as few as 25 transit,” said Kaltenegger, director of the Cornell Carl Sagan Institute. “WD 1856 b suggests that planets can survive the chaotic history of white dwarfs. Under the right conditions, these worlds could maintain favorable conditions for life beyond the time scale predicted for Earth. Now we can explore many new exciting possibilities for worlds that orbits these dead stars nuclei. “
There is currently no evidence to suggest that there are other worlds in the system, but it is possible that additional planets exist and have not yet been discovered. They could have orbits that exceed the time TESS observes a sector or overturns in such a way that transit does not occur. The white dwarf is also so small that the ability to capture transits from planets further out in the system is very low.
TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts and led by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Additional partners include Northrop Grumman, based in Falls Church, Virginia, NASA’s Ames Research Center in California’s Silicon Valley, Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, MIT’s Lincoln Laboratory and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories around the world are participants in the mission.
NASA’s Jet Propulsion Laboratory in Southern California headed the Spitzer mission for the agency’s directorate of science missions in Washington. Spitzer scientific data continues to be analyzed by the scientific community via the Spitzer Data Archive, located at the Infrared Science Archive, located at the Infrared Processing and Analysis Center (IPAC) at Caltech. Science operations were performed at the Spitzer Science Center in Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. Caltech administers JPL to NASA.
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Written by Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.