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SYFY WIRE Bad Astronomy

A Dead Star Is Vaporizing Its Planets

By Phil Plait

Astronomers are pretty sure that making planets is pretty easy when stars form. In fact, we think that most of the stars in the sky have planets orbiting them.

So what happens when the star dies? In some cases the planets will get swallowed by the star as it swells into a red giant. But the planet may even survive that. What then?

Well, not good things. The outer layers of the star blow away, exposing the star’s über-dense but intensely hot core to space. We call that a white dwarf, and any planet that survives to see it will get a white-hot blowtorch to the face as its reward.

We’ve suspected for a long time that any planets that made it through their parent star’s demise might get zapped by the leftover white dwarf, but we haven’t seen it action.

Until now, that is. The star WD 1145+017 (let’s call it WD 1145 for short) used to be similar to the Sun, long ago, and it even had a planetary system of sorts—objects at least as big as the biggest asteroids in our solar system. But it’s now a white dwarf, wreaking havoc on a massive scale: It’s so hot that it’s literally vaporizing its planets.

The smoking gun evidence of this (see what I did there?) came from observations from the Kepler spacecraft. This observatory stares at stars, looking for dips in their light as planets orbiting them block the starlight. Thousands of planets have been found using this transit method.

WD 1145 is in Kepler’s field of view, and lots of funny dips were seen in its light. Yes, this may sound familiar: The star KIC 8462852 has been in the news recently because of funny dips in its light as well, but in this case we know it’s not from aliens building gigantic megastructures around its star.

For one thing, the blips in WD 1145’s starlight are periodic (the other star shows no repeating periodicity in its diminution), occurring every 4.5 hours or so. In fact, there are clusters of them, six at least, with periods of roughly 4–5 hours. The amount of starlight blocked varies, but at one point astronomers saw a whopping 40 percent dip in light!

Mind you, a white dwarf is small, about the same size as Earth, so blocking its light is easier than it is for a “normal” star. Think of it this way: A planet the size of Earth would only block 0.01 percent of a Sun-like star’s light, but it would completely block a white dwarf!

Given the period of the dips, the objects must be orbiting WD 1145 about 800,000 kilometers out (about twice the distance of the Moon from the Earth), which is pretty close. Some of the dips in light last more than an hour, whereas a solid object would pass across the star in about a minute. That means the transiting object is huge, but not solid, or else it would completely block the light. The astronomers took a look at the star using an infrared telescope, and sure enough they saw more IR light coming from the star than you’d expect from a plain old white dwarf. That fits perfectly with it being surrounded by a lot of dust; the dust warms up due to the intense light from the white dwarf and emits infrared.

From all the data, it looks like at least one, and maybe more, planetoids are being fried by the white dwarf. The heat and light are vaporizing their surfaces, and the zapped material expands around the solid body. From the amount of light emitted by the dust, it looks like a staggering 8,000 tons of matter are boiling off every second!

The astronomers estimate the biggest of these planetoids has a mass at least equal to the protoplanet Ceres in our own solar system, which is about 1018 tons. At this rate it’ll take millions of years to completely vaporize the asteroid.

Interestingly, the dips in light seen aren’t symmetric, either; the light dims rapidly but rises more slowly. That also indicates the object doing the blocking isn’t solid, but even more, it shows the material is being blown outward by the star. As the material gets blown back, it gets farther away from the star and orbits more slowly than the planetoid. So we see the planetoid block the light from the star first because it’s orbiting faster (that’s the rapid dip), and even when the planetoid stops blocking the star, the cloud still does, so the light strengthens more slowly.

That’s very, very cool.

This process has never before been seen. We’ve seen the aftermath: Plenty of white dwarfs have been found with heavy elements polluting their surfaces. White dwarfs are very hot and have very strong gravity, so any heavy elements should sink beneath their surface very rapidly (taking about a million years to clear out; WD 1145 became a white dwarf nearly 200 million years ago, for comparison), leaving just hydrogen and/or helium behind. But many white dwarfs are seen with iron, nickel, calcium, and other elements on their surface, meaning they too must have disrupted their children planets.

But in those cases we’re seeing the events long since finished. In the case of WD 1145, we’re seeing it happen now.

Will this be the Earth’s fate when the Sun dies? Probably not. We know Mercury and Venus will be engulfed by the Sun when it turns into a red giant in about seven billion years, but they might survive the encounter, though considerably more evaporated. Earth won’t get engulfed, but having a red giant star occupying half the sky isn’t exactly conducive to staying chilly. Our planet will probably lose a bit of mass to that too.

When the Sun becomes a white dwarf, what’s left of Mercury and Venus will be much closer to it, due to millions of years of drag, being inside a star and all. They may be close enough to get fried by the white dwarf our Sun will become. The Earth will most likely be too far away, and whatever molten slag ball it has become won’t suffer that particular fate.

So, yay?

But this is fascinating, and gives us insight into how planetary systems evolve. Remember, back in 1990 we didn’t know of a single planetary system other than our own, and now we know of thousands. The very first ones were found around a neutron star, a dead star even more massive than a white dwarf, and we’ve seen them around low-mass stars, high-mass stars, young stars, old stars, multiple stars … and now we have direct evidence of them—well, whatever’s left of them—around white dwarfs, too.

All we need now is to find some around a black hole and it’ll be a clean sweep.

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