"This offers our first chance to see what a planetary system looks like after its star dies."
The James Webb Space Telescope (JWST) has already proven
itself adept at peering into the past by imaging objects at tremendous
distances, but a new breakthrough may have seen the powerful instrument act
almost like a scientific crystal ball, staring into the far future of the solar
system.
The JWST performed its prognostication when it made a
possible rare direct direction of two extrasolar planets, or
"exoplanets," orbiting two different dead stars, or "white
dwarfs."
Not only do the planets strongly resemble solar system gas
giants Jupiter and Saturn, but the white dwarfs also serve as analogs to the
sun's destiny. When the sun transforms into a white dwarf itself, the change
will likely destroy the inner solar system planets — all the way out to
Jupiter.
"Very few planets have been discovered around white
dwarf stars. What is extraordinary about these two candidate planets is that
they are more similar to planets in our outer solar system in temperature, age,
mass and orbital separation than any planets previously found," Susan
Mullally, lead author of the research, which is yet to be peer-reviewed, and an
astronomer at the Space Telescope Science Institute, told Space.com. "This
offers our first chance to see what a planetary system looks like after its star
dies."
A snapshot of our future
The planet candidates were directly observed by the JWST's
Mid-Infrared Instrument (MIRI) as they orbit the white dwarfs WD 1202-232 and
WD 2105-82. One exoplanet candidate is located at a distance from its white
dwarf host that's equal to about 11.5 times the distance between the Earth and
the sun. The other candidate sits further from its dead stellar parent, at a
distance of about 34.5 times the separation between our planet and star.
The masses of the planets are currently uncertain, with
Mullally and colleagues estimating them to be between 1 and 7 times that of
Jupiter, the most massive planet in the solar system.
When the sun exhausts its fuel supply for the nuclear fusion
processes occuring at its core in around 5 billion years, it will swell up as a
red giant. Nuclear fusion, however, will continue in its outer layers. This
will see those outer layers of our star reach out as far as Mars, swallowing
Mercury, Venus, Earth, and possibly, the Red Planet itself. Eventually, these
outer layers will cool, leaving a smoldering stellar core, now a white dwarf,
surrounded by a planetary nebula of exhausted stellar matter.
These exoplanet detections, however, hint at what could
happen to the planets beyond Mars, the gas giants Jupiter and Saturn, when the
sun dies.
"Our sun is expected to turn into a white dwarf star in
5 billion years," Mullally said. "We expect planets to drift outward,
into wider orbits, after a star dies. So, if you wind back the clock on these
candidate planets, you would expect these to have had orbital separations
similar to Jupiter and Saturn.
"If we are able to confirm these planets, they will
provide direct evidence that planets like Jupiter and Saturn can survive the
death of their host star."
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The two exoplanets as seen around their white dwarf stars by
the JWST instrument MIRI. (Image credit: Mulaney, et al, 2024) |
Further, the white dwarfs at the heart of this discovery are
polluted with elements heavier than hydrogen and helium, which astronomers call
"metals." This could hint at what will happen to the bodies in the
asteroid belt between Mars and Jupiter after the sun dies.
"We suspect that the giant planets cause the metal
pollution by driving comets and asteroids onto the surface of the stars,"
Mullally explained. "The existence of these planets strengthens the
connection between the metal pollution and planets. Since 25% to 50% of white
dwarfs show this kind of pollution, it means that giant planets are common
around white dwarf stars."
As such, any asteroids that do survive the death of the sun
could find themselves pelted at its corpse by Jupiter and Saturn.
The dual discovery is impressive beyond what it predicts for
the future of our planetary system — it also simply represents a rare
scientific achievement.
A rare direct exoplanet detection
Since the discovery of the first exoplanets in the
mid-1990s, astronomers have discovered around 5,000 worlds orbiting stars
outside the solar system. According to the Planetary Society, as of April 2020,
only 50 of these exoplanets had been discovered with direct imaging.
That is because any light from a planet at such vast
distances is usually overwhelmed by the intense light from that planet's parent
star, making directly spotting an exoplanet similar to sighting a firefly
sitting on the lit lamp of a lighthouse.
As a result, exoplanets are usually seen by the effect they
have on the light of their star, either by causing a dip in light output as
they cross, or "transit," the star's face or through a
"wobble" motion created as the planet gravitationally tugs on the
star.
"We directly imaged these two exoplanets, which means
we took their picture and are seeing the light produced by the planet
itself," Mullally said. "Most exoplanets that have been discovered
have been found using the transit method or by measuring the motion of the
star. These indirect methods tend to favor planets much closer to the star.
Direct imaging is better at finding planets farther away from the star, at
wider orbital separations."
She explained that, by spotting these planets directly, the
JWST has opened up the possibility of studying these worlds further; scientists
can now start investigating things like the composition of the planets'
atmospheres and directly measure their masses and temperatures.
Mullaly added that not everything she and her team
discovered about these exoplanets was expected, and that these quirks could
change how astronomers think about exoplanets like these in general.
Alternatively, the strange features of the targeted worlds
could offer tantalizing hints in the direction of long-sought exomoons.
"If these are planets, then it is surprising that they
are not as red in the mid-infrared as we might expect. The amount of light
collected by JWST at 5 and 7 microns is brighter than we might expect for both
exoplanet candidates given their age and how bright they are at 15
microns," Mullally concluded. "This might challenge our understanding
of the physics and chemistry of exoplanet atmospheres.
"Or maybe it means there is another source of light,
like a heated moon orbiting the planet."
The team's research is available as a preprint on the
research repository site arXiv.