Study of 'twin' stars finds some of them are planet-eaters
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[March 21, 2024]
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.
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"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.
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![](../images/032124PIX/news_g10.jpg)
An artist's impression shows a terrestrial planet in the process of
being captured by a twin star, in this handout illustration obtained
by Reuters on March 20, 2024. Intouchable, OPENVERSE/Handout via
REUTERS
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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)
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