Study of 'twin' stars finds some of them are planet-eaters

By Will Dunham

WASHINGTON (Reuters) - The planetary system that includes Earth and its sibling planets orbiting the sun has been remarkably stable during its roughly 4.5 billion years of existence. But not all planetary systems are so lucky, as shown in a new study involving "twin" stars.

An examination of 91 pairs of stars with matching sizes and chemical compositions showed that a surprising number exhibited signs of having ingested a planet, scientists said on Wednesday, likely after the planet was sent hurtling out of a stable orbit for any number of reasons.

The study looked at pairs of stars that formed within the same interstellar cloud of gas and dust - so-called co-natal stars - giving them the same chemical makeup, and were of roughly equal mass and age. These are the "twins." While the pairs are moving together in the same direction within our Milky Way galaxy, they are not binary systems of two stars gravitationally bound to each other.

A star's chemical composition changes when it engulfs a planet because it incorporates the elements that made up the doomed world. The researchers looked for stars that differed from their twin because they had higher amounts of tell-tale elements like iron, nickel or titanium indicating remnants of a rocky planet, relative to certain other elements.

"It's the elemental abundance differences between two stars in a co-natal system," said astronomer Fan Liu of Monash University in Australia, lead author of the study published in the journal Nature.

In seven of the pairs, one of the two stars bore evidence of planetary ingestion.

Possible reasons for a planet making a death plunge into its host star include an orbital disturbance caused by a larger planet, or another star passing uncomfortably close, destabilizing the planetary system, the researchers said.

"This really puts into perspective our fortuitous position in the universe," said astrophysicist and study co-author Yuan-Sen Ting of the Australian National University and Ohio State University. "The stability of a planetary system like the solar system is not a given."

The researchers used the European Space Agency's Gaia space observatory to identify the twins and used telescopes in Chile and Hawaii to determine their composition. The stars were as close as 70 light years from our solar system and as far as 960 light years away. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

The researchers said while it is most likely that their observations signaled whole planets being ingested, it was possible it was planetary building blocks consumed during the system's period of planet formation.

In their death throes, our sun and other stars like it dramatically puff up, ingesting any planets with close orbits, before collapsing into a dense, burned-out cinder called a white dwarf.

"We know that all stars like the sun will eventually become giant stars. The envelope of the sun will expand and eventually swallow Earth," Ting said.

But the stars in this study all were in the prime of their life, not nearing the end.

Instability in planetary systems may be more common than previously known, considering that about 8% of the stellar pairs studied had one star that apparently devoured a planet.

Most planetary systems should be stable because, as in our solar system, the planets are under the influence mainly of their host star, not their sibling planets, Ting said.

"But for other planetary systems with different initial conditions and configurations, this might break down, leading to very chaotic dynamics," Ting added.

The study indicates that, Ting said, "a non-negligible fraction of planetary systems are indeed unstable, meaning there are always planets being ejected in or out."

Given that only a small fraction of these wayward planets might actually be gulped by their host star rather than simply wandering the cosmos, there may be more of these planetary exiles than previously suspected.

"Understanding which planetary systems are stable or not is a long-time goal of planetary dynamics theorists," said Ting.

(Reporting by Will Dunham, Editing by Rosalba O'Brien)