Planet-eating stars could help search for other 'Earths'

Even though our sun is a boiling cauldron of gas, metal, radiation, and nuclear reactions, it's still pretty tame. Case in point: it hasn't devoured the Earth yet, unlike some other stars in the galaxy that tend to dine on the planets circling around them.

Trey Mack, a graduate student in astronomy at Vanderbilt University, has looked at two G-class (yellow) stars similar to our own sun and determined what happens to them when they begin to eat rocky planets like our Earth. By decoding the chemical composition of such stars, Mack believes that we can look out into the galaxy and more easily find solar systems that host Earth-like planets.

When a star shows evidence of having eaten planets, "we will be able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets," Mack said in a statement. "And when we find stars that lack these signatures, then they are good candidates for hosting planetary systems similar to our own."

Although suns might seem like the evil villain in this space opera -- sucking in and vaporizing the very planets that revolve around them -- the real culprits in sending the planets to their doom are other planets.

"Imagine that the star originally formed rocky planets like Earth. Further, imagine that it also formed gas giant planets like Jupiter," Mack said. "The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets.

"With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect."

So what exactly is that signature? How does a steady diet of planets affect a star's composition?

To find out, Mack -- along with co-author Simon Schuler of the University of Tampa and study supervisor, Vanderbilt Professor of Astronomy Keivan Stassun -- examined the chemical makeup of a binary pair of stars known as HD 20781 and HD 20782 using spectroscopy, which translates the chemical composition of the stars into bands of color.

What they found was that each contained elevated levels of elements including aluminum, silicon, calcium, and iron -- elements critical in the formation of Earth-like planets. This led the team to conclude that one of the stars had ingested at least 10 Earth-sized planets while the other had gobbled up at least 20 of them.

If this kind of chemical analysis proves to hold true for other solar systems in the galaxy, then the researchers will have found a way for us to easily and accurately predict which suns might have Earth-like planets circling around them and which have gobbled them all up. Obviously, stars that show chemical evidence of having gulped down rocky planets are unlikely places to look for Earth-like worlds.

What's more, Stassun believes the work could fundamentally change the way astronomers look to the heavens to find Earth-like exoplanets. "This work reveals that the question of whether and how stars form planets is actually the wrong thing to ask," he said. "The real question seems to be how many of the planets that a star makes avoid the fate of being eaten by their parent star?"