Stars, planets may see violent end
Elderly triple stars and planets outside our solar system may have chaotic end involving collisions and binary systems.
Tadpole Nebula 12,000 lightyears from Earth. Photo: UCLA/JPL-Caltech/NASA/Reuters
They leave home, switch partners and take part in violent clashes — but they are
stars and planets, not humans. The wild life of elderly triple stars and
exoplanets involve physical collisions, star-hopping planets and exotic binary
Researchers at the Technion-Israel Institute of Technology in
Haifa and the University of Colorado in Boulder used theoretical analysis and
simulations to show that the evolution of planets and stars in triple stellar
systems can lead to chaotic outcomes.
In fact, the famous star that is
the brightest in our sky – Sirius – may have had such a wild history. The
findings also suggest that stellar collisions may be as much as 30 times more
frequent than thought before.
Most stars either live alone, with no
stellar companion, or share their life with a single companion star. However,
about 15 percents of all stars have at least two additional stellar companions,
making them part of a triple system. A planet can also be hosted by a binary
stellar system, making it a component in a triple system.
of stars as they age involve major changes; a star can expand to become hundred
times larger than its original size, and then shed most of its mass through
strong winds, to ends its stellar life, leaving a remnant “white dwarf” — or end
its life in an explosion, leaving a remnant neutron star or a black
The evolution of single stars have been extensively studied over
the past century, and the evolution of binary stars, in which one star may
exchange material with its companion, have been explored for
Prof. Hagai Perets of the Technion and Dr. Kaitlin Kratter of
the University of Colorado published their work recently in two studies in The
The evolution of single stars have been studied
extensively over the last century, and the evolution of binary stars – in which
one star may exchange material with its companion – have been explored for
decades. However, by comparison, the evolution of triple stars have by and large
The Israeli-American team said that triple systems and
their evolution deserve much more attention. Single and binary systems are
generally stable systems, said Perets. Triple systems, however, are much more
fragile, and in some triple systems, even a small change in the system can drive
it into an unstable configuration, leading to a wild, chaotic dynamical
The researchers found that the mass lost from one of the
triple components as it ages may lead to a significant change in the orbits of
the triple components around each other. Many such systems then become unstable,
and the three stars may go through a wild dance, in which the stars can switch
partners. Eventually, one of the stars is ejected from the
However, since the mass-losing star is at this stage also
typically puffed up to become 100 times larger than its original size, it then
becomes a large target, and the chances for another star to collide with it
during the chaotic evolution become quite high.
Typically, Perets added,
stars collide only in a dense environments in stellar clusters, where two
unrelated stars hit each other by chance, but the probability for such
collisions is small, and outside clusters such collisions practically never
happen. “We found that when one considers triple systems, the picture is
completely different. Collisions between stars outside these dense cluster
environments can be as much as 30 times more frequent, compared with the random
collisions in cluster,” he noted.
The researchers found that triple
evolution might even be relevant for Sirius; which is accompanied by a white
dwarf in a binary system, but it has an elliptical orbit. Such a configuration
is peculiar for a binary with a white dwarf component (on a relatively close
orbit); such binaries are expected to have a much more circular orbit following
the transfer of material from a mass-losing star to its companion.
recent study showed that such a peculiar configuration could arise naturally if
the Sirius binary had once been a triple system, which evolved, become unstable,
and ejected one of the triple components, leaving behind a binary with a
peculiar orbit. “Finding out that the triple evolution scenario we studied could
provide an explanation for the Sirius puzzle, and that Sirius have had a wild
history was a fascinating thought,” said Perets.
In the second study, the
researchers looked into systems in which the triple component is not a star, but
“When the evolving star has a close planetary companion and a a
stellar companion further away, the results can be quite surprising” the
Colorado scientist said. “A planet can hop from its original orbit around its
host star, to orbit the companion star. Such a star-hopper can jump between its
original host and the companion many times, to finally settle down into a new
stable orbit around the companion.
It could then spend the rest of its
life in its new environment, or at least until the companion star also starts
losing its mass. Basically, when the neighborhood becomes bad, the planet jumps
around to a better location.”
However, the story sometimes have a worse
ending for the planet, as it may also collide with the companion star or with
its original host star during the chaotic evolution, and be destroyed rather
than be captured into a new stable orbit.