The James Webb Space Telescope (JWST) has already proven itself adept at peering into the past by imaging objects at enormous distances, but a new breakthrough may have made the powerful instrument behave almost like a scientific crystal ball behaves, staring into Earth’s distant future. solar system.
The JWST carried out its prediction when it made a potentially rare direct trajectory of two extrasolar planets, or “exoplanets,” orbiting two different dead stars, or “white dwarfs.”
Not only do the planets closely resemble the solar system’s gas giants Jupiter and Saturn, but the white dwarfs also serve as analogues for the sun’s fate. When the Sun itself turns into a white dwarf, the change will likely destroy the planets in the inner solar system – all the way to Jupiter.
“Very few planets have been discovered around white dwarf stars. What is special about these two candidate planets is that in terms of temperature, age, mass and orbital distance, they are more similar to planets in our outer solar system than any previously found planets,” says Susan. Mullaly, lead author of the study, which has yet to be peer-reviewed, and an astronomer at the Space Telescope Science Institute, told Space.com. “This provides our first chance to see what a planetary system looks like after its star dies.”
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A snapshot of our future
The planet candidates were directly observed by the JWST’s Mid-Infrared Instrument (MIRI) as they orbited the white dwarfs WD 1202-232 and WD 2105-82. One exoplanet candidate is at a distance from its white dwarf host equal to about 11.5 times the distance between Earth and the Sun. The other candidate is further from its dead star parent, at a distance of about 34.5 times the distance between our planet and the star.
The masses of the planets are currently uncertain, with Mullaly and colleagues estimating that they are between 1 and 7 times greater than that of Jupiter, the most massive planet in the solar system.
When the sun has exhausted its fuel supply for the nuclear fusion processes that will take place in its core in about 5 billion years, it will swell like a red giant. However, nuclear fusion will continue in the outer layers. This will cause the outer layers of our star to reach out to Mars and swallow 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 depleted stardust.
However, these exoplanet detections hint at what could happen to the planets beyond Mars, the gas giants Jupiter and Saturn, if the Sun dies.
“Our sun is expected to turn into a white dwarf star in five billion years,” Mullaly says. ‘We expect planets to drift outwards, into wider orbits, after a star dies. So if you turn back the clock on these candidate planets, you would expect them to have had orbital separations similar to those of Jupiter and Saturn.
“If we can confirm these planets, they will provide direct evidence that planets like Jupiter and Saturn can survive the death of their host star.”
In addition, the white dwarfs at the heart of this discovery are contaminated with elements heavier than hydrogen and helium, which astronomers call “metals.” This could indicate 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 to the surface of the stars,” Mullaly explains. “The existence of these planets strengthens the link between the metal pollution and planets. Since 25% to 50% of white dwarfs exhibit this type of contamination, this means that giant planets are common around white dwarf stars.”
As such, any asteroids that survive the Sun’s death could be pelted by Jupiter and Saturn.
The double discovery is more impressive than what it portends for the future of our planetary system – it also simply represents a rare scientific achievement.
A rare direct detection of exoplanets
Since the discovery of the first exoplanets in the mid-1990s, astronomers have discovered about 5,000 worlds orbiting stars outside the solar system. According to the Planetary Society, only 50 of these exoplanets had been discovered by direct imaging as of April 2020.
That’s because at such great distances, all the light from a planet is usually overwhelmed by the intense light from that planet’s parent star, making directly spotting an exoplanet akin to spotting a firefly perched on the illuminated lamp of a lighthouse .
As a result, exoplanets are usually thought of by the effect they have on the light from their star, either by causing a dip in light output as they cross or ‘cross’ the star, or by a ‘wobbling’ motion that is created when the star planet pulls gravitationally on the star.
“We directly imaged these two exoplanets, which means we took a picture of them and see the light produced by the planet itself,” says Mullaly. ‘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 that are much closer to the star. Direct imaging is better at finding planets further away from the star, at wider orbital separations.”
She explained that by directly spotting these planets, the JWST has opened up the opportunity to further study these worlds; scientists can now start investigating things like the composition of the planets’ atmospheres and directly measure their masses and temperatures.
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Mullaly added that not everything she and her team discovered about these exoplanets was expected, and that these oddities could change the way astronomers think about these types of exoplanets in general.
Alternatively, the strange features of the targeted worlds could offer tantalizing hints toward long-sought exomoons.
‘If these are planets, it is surprising that they are not as red in the mid-infrared as we would expect. The amount of light collected by JWST at 5 and 7 microns is brighter than we would expect for both exoplanet candidates given their age and how bright they are at 15 microns,” Mullaly concluded. ‘This could challenge our understanding of the physics and chemistry of exoplanet atmospheres.
“Or maybe it means there’s another light source, like a heated moon orbiting the planet.”
The team’s research is available as a preprint on the research repository site arXiv.