This exoplanet, a planet outside our solar system, is the size of Jupiter and is known as WD 1586 b.
This giant exoplanet zips around the star remnant, which is about the size of Earth, in a very close orbit every 34 hours. By comparison, Mercury is the closest planet to the sun in our solar system and takes 90 days to complete an orbit around the sun.
A white dwarf is what is left after a sun-like star swells up to a red giant during the star’s evolution. Red giants burn through their hydrogen fuel and expand and consume planets near their path. For example, when our sun becomes a red giant for billions of years from now, it will probably engulf Mercury and Venus – and perhaps Earth.
After the star loses its atmosphere, the only thing left is the collapsed nucleus – the white dwarf. It continues to cool for billions of years.
Finding an intact planet in such a dense orbit around a white dwarf raises questions about how it got there and how it survived the star’s evolution into a white dwarf.
The scientists believe that the planet was much further away from the host star and traveled closer after the star evolved.
Their simulations suggested that when the star became a white dwarf, the planet was kicked closer.
The study suggested a theory that large planets can survive a star’s violent evolution and arrive at a dense orbit around it afterwards.
“We believe this star died and became a white dwarf about six billion years ago – so long ago that the sun, earth and solar system had not even been formed yet,” said Ian Crossfield, study author and assistant professor of physics. and astronomy at the University of Kansas, in an email.
Although the star is just a fading glow (only one tenth as bright as our sun), this planet is probably now on a stable orbit, so it should be there for us to study and learn more about it for many years to come. “
Finding a dead solar system
NASA’s planetary hunt Transiting Exoplanet Survey Satellite (TESS) mission was launched in 2018 and has since searched for exoplanets around nearby stars. This white dwarf is one of the oldest observed by TESS.
Scientists noticed the planet as they searched through data collected by TESS.
“TESS finds a planet by looking at a star, and it measures how bright the star is continuously for weeks,” Crossfield said. “If a planet orbits the star, and if the planet passes between you and the star, some of the star’s light will be blocked. Then the star will be brighter again when the planet passes – we call this ‘transit of the planet.’
While TESS data can reveal the presence of something, it is not always clear what the object is, he said. It could be a faint star passing by, rather than a planet.
To help confirm the discovery of the planet, Crossfield used NASA’s Spitzer Space Telescope before its mission ended in January. Spitzer was designed to make infrared observations and see objects that would otherwise be invisible in visible light.
Infrared light was the key to helping scientists determine if the object was a small star or a large planet. Stars emit infrared light, but planets are colder than stars, so they do not.
“What our Spitzer data showed is that there is virtually no infrared light at all,” Crossfield said. “And the depth of these transits is identical between the TESS data and our Spitzer data set. It really puts the final nail in the coffin that this thing is almost certainly a planet rather than a star.”
Follow-up observations with ground-based telescopes, including some operated by amateur astronomers, also helped confirm the finding.
The planet is no more than 14 times the mass of Jupiter, scientists determined.
After discovering the planet, the scientists ran simulations to determine how the planet arrived so close to the star. If the red giant devoured the nearer planets in its path, this would destabilize the longer orbit of the Jupiter planet and send it to an oval orbit that would bring it close to the white dwarf, but also send it far out.
Over time, this energetic dance slowed down and led the planet into a short, tight orbit over billions of years.
When our own sun becomes a red giant, is it so possible that the Earth could survive this stellar evolution?
“About five billion years, our sun will become a white dwarf,” Crossfield said. “There are many open questions about whether planets can survive the process of a star blowing up to become a red giant, swallowing some of the inner planets and then shrinking back and just staying back as the white dwarf again.
“Can planets actually survive it – or is it impossible? And until now, there were no known planets around white dwarfs.”
Although it is unlikely that Earth will survive, “Mars, the asteroid belt, and all gas-giant planets are likely to survive and remain in altered orbits around the remnants of the Sun,” Steven Parsons said in an article accompanying the study. Parsons, an Ernest Rutherford fellow in the astronomy group at the University of Sheffield, was not involved in this study.
Could habitable planets exist around dying stars?
Given the size of this planet, it is probably a gas giant similar to Jupiter in our solar system.
“This particular planet is not a good candidate for habitability,” said Andrew Vanderburg, lead author and assistant professor of astronomy at the University of Wisconsin, Madison. “It’s big enough for it to have a suffocating atmosphere made of hydrogen and helium. So it’s not a good place for life as we know it to live.”
Researchers expect this system to continue for billions of years to come as the white dwarf continues to cool off and “enjoy a long, peaceful retirement,” Vanderburg said.
However, the discovery of a planet around a white dwarf raises questions about a uniquely habitable environment that could exist close to the light of a dying star. White dwarfs release light and heat as they cool, so a dense planet could actually be in the habitable zone of the star or the Goldilocks area, where the temperature on the planet is just right to support liquid water – and even potential life – – on the surface .
“This tells us that white dwarfs can have planets, which is something we did not know before,” Crossfield said. “There are people now looking for transiting planets around white dwarfs that could potentially be habitable. Now at least we know that some types of planets can survive and be found there, so it provides greater support and greater interest in continuing. the search for even smaller planets around these white dwarfs. “
This suggests that dead solar systems could actually host hospitable regions, Vanderburg said.
Scientists look forward to searching for smaller planets around white dwarfs in the future as well as determining more about the planet they found.
“It seems that white dwarf systems can be a pretty good place to live if your planet happens to be in the right part of the system,” Vanderburg said. “So if WD 1856 can reach it to this part of the system, then maybe other smaller planets might as well, including the rocky planets we expect to be the best places for life to exist.”