The violent events that led to the death of a star would probably drive any planet away. It is possible that the newly discovered object the size of Jupiter arrived long after the star’s death.
An international team of astronomers using NASA’s Transit Exoplanet Survey Satellite (TESS) and the retired Spitzer Space Telescope have reported that it could be the first intact planet to orbit a white dwarf orbit, 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, called WD 1856 + 534. It orbits this star ash every 34 hours, more than 60 times faster than the that Mercury orbits our Sun.
How could a giant planet have survived the violent process that transformed its parent star into a white dwarf? Astronomers have some ideas after discovering the Jupiter-sized object WD 1856 b. Credit: NASA / JPL-Caltech / NASA’s Goddard Space Flight Center
“The WD 1856 somehow got 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 process of creating the white dwarf destroys nearby planets, and everything that later approaches is usually broken by the immense gravity of the star. We still have many questions about how WD 1856 b came to its location. current without fulfilling one of these destinations “.
A paper on the system, directed by Vanderburg and including several NASA co-authors, appears in the September 17 issue of Nature and is now available online.
TESS monitors large strips of sky, called sectors, for almost a month at a time. This long look allows the satellite to find exoplanets, or worlds beyond our solar system, capturing changes in stellar brightness caused when a planet crosses ahead or transits its star.
The satellite saw WD 1856 about 80 light-years away in the northern constellation of Draco. It orbits a fresh, calm white dwarf that is 18,000 kilometers wide, 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. Finally, it expels the outer layers of gas and loses up to 80% of its mass. The remaining hot core becomes a white dwarf. Any nearby object is usually swallowed and incinerated during this process, which in this system would have included WD 1856 b in its current orbit. Vanderburg and colleagues estimate that the possible planet should have originated at least 50 times farther from its current location.
“We have known for a long time that after white dwarfs are born, small distant objects, such as asteroids and comets, can disperse into these stars. In general, the force of gravity of a white dwarf they separate them and become a waste disk, ”said co-author Siyi Xu, an assistant astronomer at the Gemini International Observatory in Hilo, Hawaii, 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 signs that the planets could also be scattered inland, but it looks like it’s the first time we’ve seen a planet that makes all the trip is intact “.
The team suggests several scenarios that could have propelled WD 1856 to 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 huge tides that dissipated its orbital energy.
“The most likely case involves several other bodies the size of Jupiter close to the original orbit of WD 1856 b,” said co-author Juliette Becker, a 51 Pegasi ba Caltech planetary science fellow in Pasadena. “The gravitational influence of such large objects could easily allow for the instability you need to bring a planet inward. But right now, we still have more theories than data points.”
Other possible scenarios involve the gradual gravitational pull of the other two stars in the system, the red dwarfs G229-20 A and B, over billions of years and an overflight of a rogue star that disrupts the system. The Vanderburg team believes these and other explanations are less likely because they require fine-tuned conditions to achieve the same effects as possible giant companion planets.
Jupiter-sized objects can occupy a huge range of masses, however, from planets only a few times more massive than Earth to stars of low mass thousands of times the mass of Earth. Others are brown dwarfs, straddling the line between the planet and the star. Scientists usually use radial velocity observations to measure the mass of an object, which can hint at its composition and nature. This method works by studying how an orbiting object pulls its star and alters the color of its light. But in this case, the white dwarf is so old that its light has become too faint and too simple for scientists to detect noticeable changes.
Instead, the team observed the system in the infrared using Spitzer, a few months before the telescope was turned off. If WD 1856 b were a brown dwarf or a low-mass star, it would emit its own infrared glow. This means that Spitzer would record brighter traffic than if the object were a planet, which it would block instead of emitting light. When the researchers compared Spitzer data with visible light traffic observations taken with the Gran Canaria Telescope in the Canary Islands, 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 probably a planet no more than 14 times the size of Jupiter. Future research and observations may confirm this conclusion.
Finding a possible world around a white dwarf prompted co-author Lisa Kaltenegger, Vanderburg, and others to consider the implications for studying atmospheres of small rocky worlds in similar situations. For example, suppose a planet the size of Earth was located in the range of orbital distances around WD 1856 where water could exist on its surface. Using simulated observations, the researchers show that NASA’s upcoming James Webb space telescope could detect water and carbon dioxide in the hypothetical world by observing only five transits.
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 could detect combinations of gases that could potentially indicate biological activity in this world in just 25 transits,” said Kaltenegger, director of the Carl Sagan Institute in Cornell. “WD 1856 b suggests that planets can survive the chaotic stories of white dwarfs. Under the right conditions, these worlds could maintain favorable conditions for life beyond the time scale predicted for Earth. We can now explore many new intriguing possibilities for worlds orbiting these dead star cores “.
There is currently no evidence to suggest that there are other worlds in the system, but additional planets may exist and have not yet been detected. They could have orbits that exceed the time that TESS observes a sector or that are tilted so that no traffic occurs. The white dwarf is also so small that the chance of catching transits from more distant planets in the system is very low.
TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Other partners include Northrop Grumman, based in Falls Church, Virginia, NASA’s Ames Research Center in Silicon Valley, California, the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, MIT’s Lincoln Laboratory, and the Space Telescope Science Baltimore Institute. More than a dozen universities, research institutes and observatories around the world participate in the mission.
NASA’s jet propulsion laboratory in Southern California managed the Spitzer mission to lead the agency’s scientific mission to Washington. Spitzer’s scientific data continues to be analyzed by the scientific community through the Spitzer Data Archive, located in the Infrared Science Archive located at Caltech’s Infrared Processing and Analysis Center (IPAC). The scientific operations were conducted at Caltech’s Spitzer Science Center. The spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. Caltech manages JPL for NASA.
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Written by Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.