Rick Robinson

Sep 28th 2020

The Case of the Disappearing Planet


A planet is nearly disappearing before our eyes. Its atmosphere is so hot from its nearby sun that it is gradually being blown off into the depths of space.

And the evaporating exoplanet, which NASA identified as GJ 3470b, is not alone in facing a catastrophic future. Another slowly disappearing planet, GJ 436b, is also losing its atmosphere to space, though at a more gradual rate.

Yet another world is facing an even more spectacular and catastrophic fate. The evaporating exoplanets may leave behind rocky cinders of themselves, still orbiting their parent stars. But WASP-12b, according to Sky &Telescope, appears doomed to total disintegration when it spirals into its parent star in a cosmically brief 3 million years.

Two planets have already disappeared: The University of California at Santa Cruz reported that a double star system called BD +20 307, located some 300 light-years from Earth, contains large quantities of heated dust — the likely result of a recent collision of exoplanets.

Planets, Planets, Everywhere

One disappearing planet might be a cosmic fluke. Nearly half a dozen disappearing planets suggests a trend — perhaps an alarming one.

The good news is that these results are a sign that there are a lot of planets out there. Northrop Grumman’s Robert Lockwood, project manager of TESS, the Transiting Exoplanet Survey Satellite said, “The single most important thing to understand about exoplanets is that they are everywhere. The number and variety of planets is far beyond anything we expected to see.”

The Extrasolar Planets Encyclopaedia expanded upon this: there are 4,150 confirmed planets beyond our solar system, orbiting in 3,083 planetary systems, 675 of which are known to have more than one planet. With so many planets being observed, it is still amazing — but not surprising — that some are headed for a spectacular end.

Shadow Dances

Perhaps the only thing more remarkable than discovering so many planets is that we have done it without ever actually seeing them. (Sometimes, in astronomy, discovery is all about the things you can’t see.)

As Lockwood explains, at our current level of technology, astronomers rely on indirect effects to detect planets beyond the solar system. The most common detection method, used by TESS, detects the slight dimming of distant stars as their planets pass in front of them. This is a bit like detecting a runner at night by noting the lights that momentarily blink out. Other detection methods are even more oblique.

Finding Doomed Worlds

Our methods of identifying doomed planets — or their aftermath — are equally oblique. As Sky & Telescope reported, the death spiral of WASP 12b was detected by timing its successive orbits. Its orbital period is gradually getting shorter. While still under investigation, the pattern strongly suggests orbital decay due to tidal forces.

In the cases of the evaporating planets, said, astronomers began by noticing an odd and suspicious pattern. Our current methods of detecting planets work best for planets orbiting close to their parent stars, so we have detected a large number of “hot” planets.

But both giant “hot Jupiters” and small “hot Earths” seem to be more common than intermediate sized “hot Neptunes.” Since large planets are believed to be composed of mostly gaseous or volatile materials, while small ones are rocky (and therefore much more heat-resistant), one prevailing theory is that hot planets, close to their parent stars, are subject to losing their atmospheres.

The idea is that Jupiter-sized hot giants are big enough to retain their atmospheres for billions of years, but smaller, Neptune-sized worlds may have their atmospheres largely eroded. And measurements show excess amounts of gas surrounding stars with “hot” exoplanets — confirming that the star is eroding away at the planet’s atmosphere, making it a disappearing planet.

Similar traces of stellar litter, for example, according to Science Alert, excess dust around BD +20 307 provided the telltale sign that something produced an enormous amount of dust in the recent past, an exoplanet collision being the most likely culprit.

Starting next year, when NASA‘s James Webb Space Telescope is launched into orbit and begins observations, exoplanet investigators will be able to observe exoplanets more directly — including those planets disappearing before our eyes.