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SYFY WIRE Bad Astronomy

A Tale of Three Jupiters, Part 4: Great Jupiter’s Ghost!

By Phil Plait

Three recent news stories have come out, all dealing with exoplanets—alien worlds—that have something in common with Jupiter. Part 1 was on a solitary starless exoplanet, Part 2 was about a double of our own Jupiter, and Part 3 was about a planet in a triple-star system. I was going to end it there, but then realized I could do another installment of this increasingly inaccurately named trilogy. So here’s the final chapter, appropriately enough about a planet that’s no longer there.

Thousands of years ago, a star died.

It was running out of nuclear fuel in its core, sending paroxysms throughout the star. It blew off a wind of gas and dust, material that had once been part of its outer layers. It did this again and again, until the white-hot core itself was exposed. This dense ball of compressed matter—a nascent white dwarf—emitted fierce amounts of ultraviolet light, exciting the atoms in the expelled material, causing it to glow.

Thus NGC 3242 was born, a new object from the ashes of the old.

This object is nicknamed the Ghost of Jupiter because it’s roughly the same apparent size as the planet when seen through a telescope. In fact, it belongs to a class of objects called planetary nebulae because they resemble planets in small ‘scopes.

NGC 3242 is about 1,000 light-years away, and a couple of light-years across—20 or more trillion kilometers—so it may be a tad larger than Jupiter physically.

The image above was taken by the Spitzer Space Telescope in 2013, and shows the nebula in infrared. What you see as red is actually light with a wavelength of 8 microns—10 times longer than what our eyes can see. At that wavelength, cold dust glows strongly, so you’re seeing the complex carbon and silicate molecules blown out by the star millennia ago. You can even see concentric circles; those are actually shells of material ejected whenever the star had one of its spasms as its fuel ran out.

Overall the nebula is circular (really spherical), but the inner core is noticeably elliptical. Why? Oh, this is where irony comes in quite strongly.

The noncircular inner regions of planetary nebulae were a mystery for a long time. I studied these objects for my master’s degree, in fact, and in 1990 we didn’t really know why the inner parts were shaped the way they are. In the paper we published, my adviser, Noam Soker, made an aside in one section that, at the time, was a weird claim: Perhaps the star had planets.

When it began to run out of fuel, the star expanded into a red giant. Planets close in would be consumed. They orbited faster than the star spun, so they would spin it up, making it rotate faster. Centrifugal force would flatten the ejected gas and dust, creating a more (American) football-shaped object. The outer parts of the nebula were ejected before this happened and would be more spherical.

Then an amazing thing happened: Five years later the first exoplanet orbiting a Sun-like star was found. Not only that, but it was a planet more massive than Jupiter, orbiting extremely close in to its star. Over the next few years more such “hot Jupiters” were found, and now we know they’re actually rather common.

Voilà! That likely explains the inner regions of elliptical planetary nebulae! Noam was right. At least partially; it’s also possible these stars were binary, and consumed their partner star. But massive close-in planets work too, and we know they exist.

That may very well have been the case for the central star of NGC 3242. Perhaps it had planets, maybe even a hot Jupiter making a tight orbit around it. It expanded, got spun up by the planet, and started emitting that flattened wind which formed the elliptical shape we see. As for the planet itself it would fall to the star’s center, evaporating fiercely, until it merged with the star itself, eaten by its parent.

Do you see the irony? The Ghost of Jupiter may actually very well be the ghost of a Jupiter-like planet!

And that, it seems to me, is a fitting way to end this four-part trilogy.

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