in a star 2,600 light-years away, a Jupiter-like exoplanet called Kepler-1658b is hurtling toward a fiery collision with its star, and could shed light on the terrible fate that awaits our cozy universe.
Astronomers would have remained blissfully ignorant of the exoplanet’s fate without one small clue: a tiny change in its orbit, revealed only by comparing more than a decade’s worth of data from several telescopes. Astrophysicist Shreyas Visapragada of the Harvard-Smithsonian Center for Astrophysics and colleagues recently published their results in Astrophysical Journal Letters.
what’s new – Astronomers have watched Kepler-1658b pass between Earth and its star about every two weeks for the past thirteen years, and note that its orbit is slowly shrinking. Each year, the gas giant takes 131 milliseconds less time to complete an orbit around its star. This means that the planet’s orbit is shrinking by a small amount each year.
If it continues, which it roughly will, Kepler-1658b will collide with its aging giant star in about 2.5 million years.
Computer models that simulate the physics of stellar systems have predicted that some planets must meet their ends by falling into their stars, but this is the first time that astronomers have actually been able to measure subtle changes in a planet’s orbit and know they’ve been sighting a planet. in the process of colliding with its star — even if the 2.5 million year time frame means we’d all miss the exciting outcome.
Here is the background – When NASA’s retired Kepler space telescope launched in 2009, on a nine-year mission to find planets orbiting other stars, the first possible exoplanet it spotted was a gas giant orbiting an advanced giant star 2,600 light-years away. A decade later, astronomers finally confirmed that the first Kepler candidate was a real exoplanet, naming it Kepler-1658b.
Kepler-1658b is essentially a much denser version of Jupiter: imagine about six planets of material packed into a sphere roughly 1.1 times the width. It is tidally tethered to its star, which means that the planet makes a complete revolution each time it finishes an orbit around the star, so the same side of the planet is always facing the star. The Moon is also tidal locked to the Earth, which is why we always see the same half of its surface.
And now it turns out that NASA’s first retired planet hunter has discovered a doomed world.
Digging into the details – Kepler-1658b is being relentlessly pulled inward by the same tidal force that is slowly pushing the Moon away from Earth. When a planet orbits a star (or a moon orbits a planet), each object orbits the mass of the other, causing it to move slightly out of shape. This is what causes tides here on Earth. This slight drag also releases energy, which can speed up the object’s orbit, boosting it higher — or slow it down, pulling it lower. Spacecraft use this trick all the time to propel themselves into higher or lower orbits.
“Tides are thought to dictate the long-term fate of hot Jupiters,” Visapragada and colleagues write in their latest paper.
Whether an object is being boosted or pulled down by tidal forces depends on how far it is from the object it is orbiting, how big both objects are, and even how fast it is spinning. In the case of the Moon, it will eventually be boosted beyond Earth’s orbit (not you Moon, it’s us, we swear). Unfortunately for Kepler-1658b, physics isn’t on the gas giant’s side. The enormous tidal force of a star more than 1.5 times the mass of our sun is gradually slowing the planet’s orbit so that it descends inward in a slow spiral.
And the poor doomed Kepler-1658b doesn’t have much leeway. Right now, it orbits its star at a distance that defies one-eighth the distance between Mercury and our sun, missing Earth (or space) with each pass.
“For hot Jupiters and other planets like Kepler-1658b that are already so close to their stars, orbital decay seems certain to culminate in destruction,” says the Harvard-Smithsonian Center for Astrophysics in a recent announcement.
Meanwhile, the same tidal forces that are slowly pulling 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 feed the volcanic surface of Io—here in our solar system, the same process may be heating parts of Kepler-1658b. This is because the planet’s surface, especially on the side facing its star, appears much brighter than it should if the planet was only reflecting starlight from the upper layers of its hot gaseous envelope. Tidal heating appears to be the most plausible explanation, say Visapragada and colleagues.
What then – The Kepler-1658 system could act as a celestial laboratory [for tidal physics] “For years to come, and with any luck, there will be more of these labs soon,” says Vissapragada in a recent announcement.
Finding doomed planets is hard and slow 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), launched in 2018—to notice the slow shrinkage of Kepler-1658b’s orbit. Recognizing signs of deadly orbital decay in other exoplanets would take a similar amount of time and a similar volume of data, but Visapragada and his colleagues say they’re there.
“We should begin to see signs of orbital decay for these planets within the next decade,” he and his colleagues wrote in their latest paper.
As for Kepler-1658b, it’s about 2.5 million years away. When the time comes, no one observing (from any alien world housing astronomers in the far future) will see the planet simply fall into the star’s outer layers and burn up, like a meteor crashing into Earth’s atmosphere. Instead, the same tidal forces that sealed its fate likely ripped the planet apart shortly before it finally did. Something similar may have happened to the long-dead moons of planets like Saturn, which now form parts of the planet’s famous ring system.
Meanwhile, we Earthlings can see a glimpse of the fate of our home world in the inevitable demise of Kepler-1658b.
“Death by a star is a fate believed to await many worlds, and could be a final farewell on Earth billions of years from now as our sun ages,” says the Astrophysics Center.
Towards the end of their lives, most stars bulge 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 the late-life Kepler-1658b swelling, leaving it about three times more massive than the sun. And about half as dense. At this point, the tidal interaction with the Sun will begin to bring the Earth closer.
What happens at that point is hard to predict. It’s possible that tides will eventually push the Earth toward the sun, but it’s also possible that with the sun losing some of its massive mass, the energy released from this process could offset the merciless tidal pull – saving our planet from a grim fate.
“The ultimate fate of the Earth is somewhat unclear,” says Visapragada.
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