Astronomers discover a star dragging its giant exoplanet into a death spiral

In a star 2,600 light-years away, a Jupiter-like exoplanet called Kepler-1658b is heading for a fiery collision with its star, and it could shed light on the terrible fate that awaits our own cozy world.

Astronomers would have remained blissfully unaware of the exoplanet’s fate had it not been for one small clue: a tiny shift in its orbit, revealed only by comparing more than a decade of data from multiple telescopes. Astrophysicist Shreyas Vissapragada of the Harvard-Smithsonian Center for Astrophysics and his colleagues recently published their findings in the Astrophysical Journal Letters.

What’s new – Astronomers have watched Kepler-1658b pass between Earth and its star about once every two weeks for the past thirteen years, and they’ve noticed its orbit slowly shrinking. Each year, it takes 131 milliseconds less for the gas giant to complete a lap around its star. This means that the planet’s orbit tightens just a tiny bit each year.

If it continues, which it almost certainly will, Kepler-1658b will collide with its aging subgiant star in about 2.5 million years.

Computer models that simulate the physics of star systems have predicted that some planets should get their way by falling into their stars, but this is the first time astronomers have been able to measure tiny changes in the planet’s orbit. a planet and know they were witnessing a planet crashing into its star – even if the 2.5 million year time frame means we’ll all miss the thrilling conclusion.

Here is the background – When NASA’s now-retired Kepler space telescope was launched in 2009, on a nine-year mission to find planets orbiting other stars, the very first possible exoplanet he spotted was a gas giant orbiting an aging subgiant star 2,600 light-years away. A decade later, astronomers finally confirmed that Kepler’s first candidate was a real exoplanet, and they dubbed it Kepler-1658b.

Kepler-1658b is essentially a denser version of Jupiter: imagine about six Jupiters of material packed into a ball about 1.1 times wider. It is tidally locked to its star, meaning the planet rotates one full turn each time it completes a lap around the star, so the same side of the planet always faces the star. ‘star. The Moon is also tidally locked to Earth, which is why we always see the same half of its surface.

And now it turns out that the first exoplanet discovered by NASA’s retired planet hunter is also a doomed world.

This artist’s illustration shows what Kepler-1658b might look like in this final phase of its life.Gabriel Perez/Instituto de Astrofisica de Canarias

Dig into the details — Kepler-1658b is being pulled inexorably inward by the same tidal force that is slowly pulling the Moon away from Earth. When a planet orbits a star (or a moon orbits a planet), each object’s gravity pulls on the mass of the other object, stretching it a little out of shape. This is what causes the tides here on Earth. And that slight tugging also releases energy, which can speed up an object’s orbit, propelling it higher – or slow it down, pulling it lower. Spaceships use this trick all the time to push themselves to higher or lower orbits.

“The long-term fate of hot Jupiters is thought to be dictated by the tides,” Vissapragada and colleagues wrote in their recent paper.

Whether an object is propelled up or pulled down by tidal forces depends on its distance from the object it is orbiting, the size of both objects, and even the speed at which it is orbiting. turned. In the case of the Moon, it will eventually be blasted straight out of Earth’s orbit (it’s not you, Moon; it’s us, we swear). Unfortunately for Kepler-1658b, however, physics is not on the gas giant’s side. The immense tidal force of a star 1.5 times the mass of our Sun gradually slows the planet’s orbit so that it falls inward in a slow spiral.

And the poor doomed Kepler-1658b doesn’t have much leeway. Right now, it’s orbiting its star one-eighth the distance between Mercury and our Sun, defying death, and it’s losing ground (or space) with each pass.

“For hot Jupiters and other planets like Kepler-1658b that are already very close to their stars, orbital decay seems certain to end in destruction,” the Harvard-Smithsonian Center for Astrophysics said in a recent announcement.

Meanwhile, the same tidal forces that are slowly dragging the gas giant to its doom are also roasting it from the inside. Just as tides keep the interiors of icy moons like Europa and Enceladus warm — and fuel the volcanic landscape of Io — here in our own solar system, the same process can heat parts of Kepler-1658b. This is because the surface of the planet, especially the side facing its star, appears brighter than it should be if the planet were simply reflecting starlight from the upper layers of its hot, gaseous envelope. . Tidal warming appears to be the most plausible explanation, Vissapragada and colleagues say.

And after – “The Kepler-1658 system can serve as a celestial laboratory [for tidal physics] for years to come,” Vissapragada says in a recent announcement, “And hopefully there will soon be many more of these labs.

Finding doomed planets is slow and painstaking work. It took thirteen years of close observation – first with Kepler and some of the most powerful telescopes here on Earth, then with NASA’s Transiting Exoplanet Survey Satellite (TESS), launched in 2018 – to notice the slow shrinking of the orbit of Kepler-1658b. Recognizing signs of deadly orbital decay in other exoplanets will take similar time and a similar volume of data, but Vissapragada and his colleagues say they’re getting there.

“We should begin to see signs of orbital decay for these planets over the next decade,” they and colleagues write in their recent paper.

As for Kepler-1658b, it has about 2.5 million years left. When the time comes, anyone watching (whatever alien world hosting astronomers in the distant future) won’t see the planet simply falling into the star’s outer layers and burning up, like a meteor falling into Earth’s atmosphere. Instead, the same tidal forces that sealed its fate will likely tear the planet apart shortly before it takes the final plunge. Something similar likely happened to the long-dead moons of planets like Saturn, which are now part of the planet’s famous ring system.

Meanwhile, we Earthlings could see a glimpse of our homeworld’s fate in the inevitable demise of Kepler-1658b.

“Star-death is a fate meant to await many worlds, and it could be Earth’s final goodbye in billions of years as our Sun ages,” says the Astrophysics Center.

Towards the end of their life, most stars swell outward. In about 5 billion years, for example, our Sun will expand until it engulfs what is now the inner solar system – a much larger version of Kepler-1658b’s late swelling, leaving it about three times wider than the Sun. and about half as dense. At this point, the tidal interaction with the Sun will begin to pull the Earth closer together.

What happens then is a little harder to predict. It is possible that the tides will eventually pull the Earth towards the Sun, but it is also possible that as the aging Sun loses some of its immense mass, the energy released by this process could offset the inexorable pull of the tides – saving our planet from a dark fate. .

“Earth’s ultimate fate is unclear,” says Vissapragada

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