For the first time, a planet has been discovered orbiting a white dwarf, also known as a dead star.
This exoplanet, a planet outside of our solar system, is the size of Jupiter and it’s 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 one orbit around the sun.
This dying star is located 80 light-years from Earth in the Draco constellation.
The study published Wednesday in the journal Nature.
A white dwarf is what remains 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, consuming any planets near their path. For example, when our sun becomes a red giant billions of years from now, it will likely engulf Mercury and Venus – and perhaps Earth.
After the star loses its atmosphere, all that remains is the collapsed core – the white dwarf. It continues to cool for billions of years.
Finding an intact planet in such a close 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 researchers believe the planet was much further away from its host star and migrated closer after the star evolved.
Their simulations suggested that when the star became a white dwarf, the planet was kicked closer in.
The study suggested a theory that large planets can survive the violent evolution of a star and arrive at a close orbit around it afterward.
“We think this star died and became a white dwarf roughly six billion years ago – so long ago that the Sun, Earth, and solar system hadn’t even been formed yet,” said Ian Crossfield, study coauthor and assistant professor of physics and astronomy at the University of Kansas, in an email.
“Though the star is just a fading ember (only one-ten-thousandth as bright as our Sun) this planet is now likely 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 planet-hunting Transiting Exoplanet Survey Satellite (TESS) mission launched in 2018 and has been seeking out exoplanets around nearby stars ever since. This white dwarf is one of the oldest observed by TESS.
Researchers noticed the planet while searching 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 is orbiting the star, and if the planet passes between you and the star, some of that star’s light is going to be blocked. Then the star will get brighter again as the planet passes – we call this the ‘transit’ of the planet.”
While TESS data can reveal the presence of something, it’s not always clear what that object is, he said. It could be a dim star that passed by, rather than a planet.
To help confirm the discovery of the planet, Crossfield used NASA’s Spitzer Space Telescope before its mission came to an end in January. Spitzer was designed to make infrared observations and see objects that would be otherwise invisible in visible light.
Infrared light was key in 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 don’t.
“What our Spitzer data showed is there’s basically no infrared light at all,” Crossfield said. “And the depths of these transits are identical between the TESS data, and our Spitzer datasets. That really put the final nail in the coffin that this thing is almost certainly a planet, rather than a star.”
Follow-up observations by ground-based telescopes, including some run by amateur astronomers, helped confirm the finding as well.
The planet is no more than 14 times the mass of Jupiter, the researchers determined.
After discovering the planet, the researchers ran simulations to determine how the planet arrived so close to the star. If the red giant devoured the closer planets in its path, this would destabilize the farther orbit of the Jupiter-size planet, sending it into an oval orbit that would bring it close to the white dwarf, but also send it far out.
Over time, this energetic dance slowed, leading the planet into a short, tight orbit over billions of years.
When our own sun becomes a red giant, is it possible that Earth could survive this stellar evolution?
“In around five billion years our Sun will become a white dwarf,” Crossfield said. “There’s a lot of open questions about whether planets can survive the process of a star inflating up to become a red giant, swallowing up some of the inner planets, and then shrinking back down and just being left over as the white dwarf again.
“Can planets actually survive that – or is that impossible? And until now, there weren’t any known planets around white dwarfs.”
While it’s not likely that Earth will survive, “Mars, the asteroid belt and all the gas-giant planets will probably survive and stay in altered orbits around the Sun’s remains,” said Steven Parsons in an article accompanying the Nature 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’s likely a gas giant similar to Jupiter in our solar system.
“This particular planet is not a good candidate for habitability,” said Andrew Vanderburg, lead study author and assistant professor in the astronomy department the University of Wisconsin, Madison. “It’s big enough that it must have a smothering atmosphere made of hydrogen and helium. So it’s not a good place for life as we know it to live.”
The researchers expect this system to persist for billions of years to come as the white dwarf continues to cool and “enjoy a long, peaceful retirement,” Vanderburg said.
However, the discovery of a planet around a white dwarf raises questions about a unique habitable environment that could exist close to the light of a dying star. White dwarfs release light and heat as they cool down, so a close 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 potentially life – on the surface.
“This tells us white dwarfs can have planets, which is something we didn’t know before,” Crossfield said. “There are people who now are looking for transiting planets around white dwarfs that could be potentially habitable. Now we at least know some kinds of planets can survive and be found there, so that gives greater support and greater interest in continuing the search for even smaller planets around these white dwarfs.”
It suggests that dead solar systems could actually host hospitable regions, Vanderburg said.
Recently, there has been a lot of research focused around the idea of searching for life on planets that could orbit white dwarfs. Now that astronomers have found such a planet, and one located in the so-called “sweet spot” around the star, it opens up a new field of exoplanet research.
The researchers 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 like white dwarf systems may 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 make it to this part of the system, then maybe other, smaller planets could as well, including the rocky planets we expect to be the best places for life to exist.”