Astronomers find Jupiter-like planet orbiting white dwarf star, in potential glimpse at our Solar System’s future

2021-10-14

A gas giant planet circling the remnants of a dead star has given astronomers a glimpse of what our Solar System might look like billions of years down the track.

Key points:

• Astronomers have found the first planet-star pair that looks like how our Solar System is predicted to end

• In about 5 billion years, the Sun will swell up and swallow the inner planets

• The finding shows Jupiter could survive the Sun’s evolution into a red giant star

The detection is the first to spot a Jupiter-like planet orbiting a white dwarf at a distance we might expect when a star runs out of fuel and dies.

An international team of astronomers, including Joshua Blackman at the University of Tasmania, report their celestial revelation in the journal Nature today.

“It was a very serendipitous discovery,” Dr Blackman said.

“This system is kind of a window into the possible future of the Solar System.”

Jonti Horner, an astronomer at the University of Southern Queensland who was not involved in the study, agreed.

“It’s showing us that when the Sun goes through this process, the giant planets are likely far enough away that they would survive.”

What’s in store for the Sun?

Astronomers don’t know for certain how the Sun will behave over the next 10 billion years or so, but they have a pretty good idea.

And it’s a fate destined for the vast majority of stars in our galaxy, the Milky Way.

Right now, the Sun, which is a pretty typical “main sequence” star, is 4.6 billion years old. The heat and light it emits is produced as it fuses hydrogen into heavier elements.

But eventually, in around 5 to 6 billion years, its hydrogen inventory will run out.

The Sun’s core will contract and collapse, and its outer layers will puff up as it evolves into a red giant, Dr Blackman said.

The surface of the swollen Sun will slowly but inexorably start encroaching on the Solar System.

And it’s bad news for Earth.

“It will engulf the inner planets, so Mercury and Venus, even Earth, will likely be destroyed,” Dr Blackman said.

“But Mars and the gas giants further out will survive. That’s the general predicted model of what’s going to happen.”

After another few hundred million years or so, the Sun will shrug its fluffy envelope into space, leaving behind a dead, dense core — a white dwarf.

But planets tipped to ride out the Sun’s red giant phase may still be there with it. They just won’t be as close.

As its mass will be less than its hydrogen-burning heyday, the white dwarf’s gravitational pull won’t be as strong, and any planets still remaining will be orbiting further out than before.

And this is what Dr Blackman and his colleagues found happening in another solar system for the first time.

EOver around 150 million years, a star similar to the Sun evolved into a white dwarf.

How to see ‘dark’ objects in space

Many distant stars can’t be “seen” using traditional telescopes, so they and other dim objects are detected using a technique called gravitational microlensing.

It’s based on the idea that the gravitational field of an object — such as a white dwarf — distorts the light shining from a star behind it, like a lens.

If the stars literally align with Earth, astronomers see the light from the furthest star intensify and smear into a curve, called an Einstein ring, as it bends around the white dwarf.

As the stars move out of alignment, the Einstein ring fades.

The white dwarf and its gas giant, which sit around 6,500 light-years away, were first spied this way in 2010 by the telescope at New Zealand’s Mount John Observatory, but it would be years before astronomers worked out exactly what they’d found.

By the time Dr Blackman joined the project in 2016, more telescopes had observed the lensing event, including the much larger and more powerful Keck Telescope on Mauna Kea in Hawaii.

“We were expecting the star to be like our Sun, a main sequence star, which is what we typically see,” he said.

“So when we didn’t see that, we spent a long time trying to figure out what we did wrong … before we could quite confidently say it was a white dwarf, about half the mass of the Sun.”

They also saw “bumps” in the Einstein ring of curved light, he added — signs of a planet lapping the white dwarf, closer than Jupiter does the Sun today, but within the expected distance of a gas giant that survived its star’s death.

A different class of exoplanet

Gravitational microlensing hasn’t found nearly as many exoplanets as other detection methods, purely because it relies on the chance alignment of stars.

Astronomers don’t know when those events will transpire, so they point telescopes into space and wait for it to happen.

But gravitational microlensing can pick up exoplanets that other methods can’t, Professor Horner says.

The transit method, where telescopes pick up momentary but regular dips in a star’s luminosity as an orbiting planet blocks the light, is good at finding bright stars with big planets orbiting very close.

The radial velocity method is also biased towards big planets circling near their star, because it detects the ever-so-slight wobble of a star caused by a planet’s gravitational tug.

Gravitational microlensing can pick up dead stars that emit little light, and have planets sitting further out.

“Think of it like a census, and you’re trying to understand the breadth of humanity,” Professor Horner said.

“The transit method is really good at finding children who are at nursery, the radial velocity method’s good at finding children at primary school.

“And this is a method that lets you find retirees.”

More telescopes, more observations

It’s not the first time a Jupiter-like planet has been spotted orbiting a white dwarf. A handful have been found, such as this one reported last year, Dr Blackman said.

“But none of those resemble what we expect to happen.

“Most are very close to their host star, like a Jupiter-mass planet 10 times closer than Mercury, whereas our planet is in alignment with the traditional expectation of what’s going to happen in the Solar System.”

More Jupiter-like planets will be found whizzing around the dead, dim remnants of their star in the coming decades, he added.

The upcoming Roman Space Telescope, earmarked for launch in the mid-to-late-2020s, will hunt for exoplanets using gravitational microlensing.

The Hubble Space Telescope and soon-to-be-launched James Webb Space Telescope could be enlisted to help too, Dr Blackman said.

“They can see much deeper into the sky, and we’re hoping we can get a direct detection of the white dwarf in the future.”

abc.net.au, 14 October 2021
; https://www.abc.net.au