circling the drain

Kepler's first exoplanet is plummeting to its doom in slow motion

Kepler-1658b’s orbit is getting a little shorter – and therefore a little closer to the blazing surface of its star – every year.

Gabriel Perez/Instituto de Astrofisica de Canarias

In a star system 2,600 light years away, a Jupiter-like exoplanet called Kepler-1658b is spiraling toward 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 ignorant of the exoplanet’s fate without a tiny clue: a minuscule change in its orbit, revealed only by comparing more than a decade of data from several 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 last thirteen years, and they noticed that its orbit is slowly shrinking. Every year, it takes 131 milliseconds less time for the gas giant to complete a lap around its star. That means the planet’s orbit tightens by just a smidgen every year.

If it keeps going, 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 meet their ends by falling into their stars, but this is the first time astronomers have actually been able to measure tiny changes in a planet’s orbit and know they were witnessing a planet in the process of crashing into its star — even if the 2.5-million-year timeframe means we’ll all miss the thrilling conclusion.

Here’s The Background — When NASA’s now-retired Kepler Space Telescope launched in 2009, on a nine-year mission to find planets orbiting other stars, the very first possible exoplanet it 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 basically a denser version of Jupiter: imagine about six Jupiters worth of material packed into a ball about 1.1 times as wide. It’s tidally locked to its star, meaning that the planet rotates one full turn every time it finishes a lap around the star, so the same side of the planet is always facing the 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 NASA’s retired planet-hunter discovered is also a doomed world.

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

Digging Into The Details — Kepler-1658b is being pulled inexorably inward by the same tidal force that’s slowly pushing the Moon away from Earth. As a planet orbits a star (or as a moon orbits a planet), each object’s gravity tugs on the other object’s mass, stretching it a little out of shape. That’s what causes tides here on Earth. And that slight tugging also releases energy, which can speed up an object’s orbit, boosting it higher — or slow it down, pulling it lower. Spacecraft use this trick all the time to nudge themselves into higher or lower orbits.

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

Whether an object gets boosted up or dragged down by tidal forces depends on how far it is from the object it’s orbiting, how big both objects are, and even how fast it’s rotating. In the Moon’s case, it will eventually get boosted right out of Earth’s orbit (it’s not you, Moon; it’s us, we swear). Unfortunately for Kepler-1658b, however, physics isn’t on the gas giant’s side. The immense tidal force of a star 1.5 times the mass of our Sun is gradually slowing the planet’s orbit so that it falls inward in a slow spiral.

And poor doomed Kepler-1658b doesn’t have much leeway. At the moment, it circles its star at a death-defying one-eighth the distance between Mercury and our Sun, and it’s losing ground (or space) with every pass.

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

In the meantime, 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 interior of icy moons like Europa and Enceladus warm — and power the volcanic hellscape of Io — here in our own Solar System, the same process may be heating up parts of Kepler-1658b. That’s because the planet’s surface, especially on the side facing its star, seems brighter than it should be if the planet were just reflecting starlight from the upper layers of its hot, gassy envelope. Tidal heating seems like the most plausible explanation, say Vissapragada and his colleagues.

What’s Next — “The Kepler-1658 system can serve as a celestial laboratory [for tidal physics] for years to come,” says Vissapragada in a recent announcement, “And with any luck, there will soon be many more of these labs.”

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

“We should begin to see hints of orbital decay for these planets within the next decade,” he and his colleagues write in their recent paper.

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

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

“Death-by-star is a fate thought to await many worlds, and it could be the Earth’s ultimate adios billions of years from now as our Sun grows older,” says the Center for Astrophysics.

Toward the end of their lives, most stars swell outward. In about 5 billion years, for instance, our Sun will expand until it swallows up what’s now the inner Solar System — a much larger version of Kepler-1658b’s late-in-life swelling, which leaves it about three times as wide as the Sun and about half as dense. At that point, tidal interaction with the Sun will start to draw Earth closer.

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

“The ultimate fate of the Earth is somewhat unclear,” says Vissapragada

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