Of all the strange worlds in our Milky Way galaxy, some of the most mysterious are those hanging around white dwarf stars.
These are not regular stars busy smashing atoms in their cores, but the ultra-dense remains of Sun-like stars that have undergone their death sequences – puffing up into enormous red giants before shedding their outer layers and collapsing into dense stellar cores.
This is what is going to happen to the Sun in about 5 billion years, so obviously any exoplanets in white dwarf orbit are of intense interest to scientists hoping to scry into the eventual fate of the Solar System.
Now, using JWST, astronomers have just obtained humanity’s first glimpse inside the atmosphere of the giant planet WD 1856b, which orbits a white dwarf – and found it far hotter than anyone expected. Their findings have been published in Nature.
“The instant we saw WD 1856b’s spectrum, which shows a colossal drop in the effective size of the planet at longer infrared wavelengths, we were like ‘Wow! What the heck is going on here?'” astronomer Ryan MacDonald, of the University of St Andrews in the UK, told ScienceAlert.
“The JWST transit spectrum of WD 1856b is unlike any other planet we’d seen before.”
White dwarf stars are among the most extreme objects in the Universe. They’re what’s left when a star up to eight times the mass of the Sun shuffles off the main-sequence coil, a dead lump that shines only with residual heat for trillions of years.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>We know roughly how the transformation goes. As the star approaches the total depletion of its nuclear fuel, it puffs up to many hundreds of times its original size.
Eventually, that puffy envelope will expand and dissipate into nothing, while the core, no longer supported by the outward pressure of fusion, collapses under gravity.
“We were like ‘Wow! What the heck is going on here?'” – Ryan MacDonald, astronomer
The resulting white dwarf is extremely dense, packing up to 1.4 times the mass of the Sun into an object that is about the same size as Earth.
For the Solar System, astronomers think this process is going to be very disruptive, with a red giant phase potentially expanding the Sun out to the orbit of Mars.
“When the Sun becomes a red giant, Mercury and Venus will be destroyed. The fate of Earth is borderline (its survival depends on fine details of the stellar model). But the outer planets, such as Jupiter and Saturn, will almost certainly survive the death of the Sun,” MacDonald explained in an email.
“When the Sun dies, and its core is left behind as a white dwarf, the lost mass will alter the orbits of the surviving planets and cause them to move outwards. Because the gravitational tug of the white dwarf will be far less than the Sun, the Solar System will be less bound, and it will be easier for dynamical interactions to move planets.”
Many exoplanets have been found hanging around white dwarfs, which naturally raises the question of how they survived the star’s death throes.
But examining a white dwarf exoplanet’s atmosphere is a lot harder than it is for a living star.
Astronomers study exoplanet atmospheres when planets pass between us and their stars. As the starlight filters through the atmosphere, it picks up signatures of the gases within, allowing astronomers to identify what the atmosphere is made of.
Most stars are a lot bigger than their planets. WD 1856b, located 82 light-years away, is seven times bigger than the white dwarf it orbits.
“Normally, the atmosphere of a transiting planet lies completely on the disc of its star during a transit, but for WD 1856b, only part of the atmosphere covers the white dwarf at any one time,” MacDonald said.
“This system has the highest known transit depth of all exoplanets (56 percent) and the transit only lasts 8 minutes. The white dwarf is also far dimmer than the main-sequence stars we normally observe. So this is truly an extremely different system to those we usually study with JWST.
“We had to develop new models and completely rethink how to analyze JWST spectra for this bizarre planetary system around a dead star.”
It wasn’t the contents of the exoplanet’s atmosphere, but its temperature, that stunned the research team.
They expected its temperature to be around -113 degrees Celsius (-171 degrees Fahrenheit), similar to Jupiter, which has a similar size and orbit to WD 1856b.
Instead, its temperature hovered around 126 degrees Celsius (259 degrees Fahrenheit).
“We were left wondering how a planet orbiting an old, dim white dwarf – which has been cooling ever since the star it formed from died 5.4 billion years ago – could be so warm?” MacDonald said.
Giant planets cool at predictable rates over billions of years. That means their present-day temperatures preserve clues to the events that heated them in the past.
The researchers were also surprised to find that, even though WD 1856b is just a little bit smaller than Jupiter, it is seven times more massive.
Together, the planet’s surprising temperature and mass allowed the researchers to reconstruct its history.
They found that the unusual world must have reached its maximum temperature billions of years after the star became a white dwarf. That timing suggests the white dwarf was not responsible; something else must have reheated it long after the stellar transformation.
One possible culprit is a nearby binary star, whose tidal influence could have both heated the exoplanet and sent it migrating inwards from a more distant orbit to its current position.
“Our results show that giant planets like Jupiter can have a ‘second life’ after the death of their star, with the planet moving close-in, being reheated, and experiencing changes in their atmospheric chemistry,” MacDonald said.
“Stellar death is not the end, it is a new chapter in the life of planets like Jupiter.”
Related: Scientists Figured Out When And How Our Sun Will Die, And It Will Be Epic
We still don’t know what that will mean for Earth. Some models see our world engulfed by the Sun, others do not. If it survives engulfment, we don’t know what the redistribution of solar mass will do to Earth’s orbit.
Humanity is unlikely to last long enough to see it, but by then, maybe new life will have emerged. Studying worlds like WD 1856b could show us what the odds of that might be.
“Today’s results show we can measure what the atmospheres of planets closely orbiting white dwarfs are made of, which opens an exhilarating new field of post-main-sequence planetary atmospheres,” MacDonald said.
“Ultimately, the dream would be to find a rocky planet orbiting a white dwarf, perhaps even in its habitable zone. Because what could be more poetic than searching for life around a dead star?”
The research has been published in Nature.
This article was fact-checked by Carly Cassella and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
