Stars That Eat Planets for Lunch

In the last couple of weeks, astronomers have announced the discovery of two stars that show evidence of having eaten some of their own Earth-like planets. Today came the announcement of the discovery of a star with planets that is growing larger and larger and is going to be eating its inner planets in the next couple of hundred million years.

These kinds of star "cannibal activity" are not that rare and should not cause us undue distress. It's business as usual in the Milky Way Galaxy, but what's new is that we are getting direct evidence for activities that earlier we only predicted from theory.

The two stars that already finished their meal a long time ago are known by their catalog numbers, HD20781 and HD20782. These stars belong to the same star system, and formed from the same original cloud of cosmic raw materials. This is the first "binary star" system where astronomers have discovered planets around each star. One has two Neptune-sized planets close by, the other has a Jupiter-sized planet whose orbit is not a nice circle but a stretched oval-shape.

Astronomers took a look at what the two stars are made of -- something we can learn by looking at the details in the colors of light from them. Every different element leaves its own "fingerprints" in the colors of light. Because the two stars formed at the same time, from the same "cosmic womb," we have more information about what they were like at the beginning.

According to Trey Mack, of Vanderbilt University, a graduate student who did the detailed analysis, both stars show evidence of elements in their atmospheres that were most likely not there when the two stars formed. Instead they are tell-tale signs of the stars having eaten planets made of rock. One star seems to have eaten the equivalent of 20 Earths, while the other consumed "only" ten Earths earlier in its history.

Astronomers have thought for a while that big "bully" planets like Jupiter and Neptune can, under the right circumstances, move inward and force smaller planets (like our Earth) to approach their stars until they are consumed. We are fortunate that this did not happen in the case of our solar system, and the Earth has a stable, undisturbed orbit which has allowed this blog's readers to evolve here. But it's getting clearer and clearer that not all star system will have the same history as ours did.

The other work deals with an act of cannibalism that happens late in the life of a star. All stars, at some point, have a "life-crisis" when their first fuel for making energy is all used up and the star has to adjust by briefly swelling up into a red-colored giant. The news is that astronomers have discovered that one of the stars around which the Kepler space mission has found three planets is on its way toward this period of swelling up. The star is known as Kepler 56, and the way in which it has started to swell up is telling us that it will eventually swallow its two inner planets, leaving only its outermost planet as survivor.

The devastation will not occur until more than a hundred million years have gone by, so it's not an immediate tragedy. We have seen many stars that swell up like this, but none of them have been known to possess planets. Kepler 56's worlds are therefore the first planets known to be orbiting other stars whose doom we can now predict. (By the way, in case you are worried, our own Sun will also have such a crisis and will swell up. But this is not happening for another five or six billion years, so I am not ready yet to include this eventuality on my home insurance policy.)

The image, by the way, is an artist's impression of an Earth-like planet being torn apart and then "eaten" by a Sun-like star.