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Twin stars could doom planets

planetary collision

This artist's concept illustrates an imminent planetary collision around a pair of double stars. NASA's Spitzer Space Telescope found evidence that such collisions could be common around certain types of tight binary star systems.

  • Dust cloud spotted circling stellar twins
  • Could be the remains of planetary collisions

Tight double-star systems might not be the best places for life to spring up, according to a new study using data from NASA’s Spitzer Space Telescope.

The infrared observatory spotted a surprisingly large amount of dust around three mature, close-orbiting star pairs. Where did the dust come from? Astronomers say it might be the aftermath of tremendous planetary collisions.

“This is real-life science fiction,” said Jeremy Drake of the Harvard-Smithsonian Centre for Astrophysics, Cambridge, Mass. “Our data tell us that planets in these systems might not be so lucky—collisions could be common.”

“It’s theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed.”

Drake is the principal investigator of the research, published in the August19 issue of the Astrophysical Journal Letters.

The particular class of binary, or double, stars in the study are about as snug as stars get. Named RS Canum Venaticorums, or RS CVns for short, they are separated by only about 3.2 million kilometres (2 million miles), or two percent of the distance between Earth and our Sun.

The stellar pairs orbit around each other every few days, with one face on each star perpetually locked and pointed toward the other.

binary star

Artist's concept of a tight pair of stars and a surrounding disc of dust—most likely the shattered remains of planetary smash-ups. Using NASA's Spitzer Space Telescope, scientists found dusty evidence for such collisions around three sets of stellar twins.

Colliding planets

The close-knit stars are similar to the Sun in size and are probably about a billion to a few billion years old—roughly the age of our Sun when life first evolved on Earth.

But these stars spin much faster, and, as a result, have powerful magnetic fields, and giant, dark spots. The magnetic activity drives strong stellar winds—gale-force versions of the solar wind—that slow the stars down, pulling the twirling duos closer over time.

And this is where the planetary chaos may begin.

As the stars cosy up to each other, their gravitational influences change, and this could cause disturbances to planetary bodies orbiting around both stars. Comets and any planets that may exist in the systems would start jostling about and banging into each other, sometimes in powerful collisions.

This includes planets that could theoretically be circling in the double stars’ habitable zone, a region where temperatures would allow liquid water to exist.

Though no habitable planets have been discovered around any stars beyond our Sun at this point in time, tight double-star systems are known to host planets; for example, one system not in the study, called HW Vir, has two gas-giant planets.

“These kinds of systems paint a picture of the late stages in the lives of planetary systems,” said Marc Kuchner, a co-author from NASA Goddard Space Flight Centre. “And it’s a future that’s messy and violent.”

Spitzer space telescope

An artist's impression of the Spitzer Space Telescope, which studies the cosmos at infrared wavelengths.

Not a fluke

Spitzer spotted the infrared glow of hot dusty discs, about the temperature of molten lava, around three such tight binary systems. One of the systems was originally flagged as having a suspicious excess of infrared light in 1983 by the Infrared Astronomical Satellite.

In addition, researchers using Spitzer recently found a warm disc of debris around another star that turned out to be a tight binary system.

The astronomy team says that dust normally would have dissipated and blown away from the stars by this mature stage in their lives. They conclude that something—most likely planetary collisions—must therefore be kicking up the fresh dust.

In addition, because dusty discs have now been found around four, older binary systems, the scientists know that the observations are not a fluke. Something chaotic is very likely going on.

If any life forms did exist in these star systems, and they could look up at the sky, they would have quite a view. Marco Matranga, first author of the paper, from the Harvard-Smithsonian Centre for Astrophysics and now a visiting astronomer at the Palermo Astronomical Observatory in Sicily, said, “The skies there would have two huge suns, like the ones above the planet Tatooine in Star Wars”.

Adapted from information issued by SAO / NASA / JPL-Caltech.

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Smallest known star duo confirmed

Artist's impression of the binary star system known as HM Cancri

About 1,600 light-years away, in a binary star system known as HM Cancri, two dense white dwarf stars orbit each other once every 5.4 minutes, based on data from the Keck Observatory. This artist's rendition shows the dance of these dead stars and the resulting gravitational waves (which would actually be invisible).

Astronomers have identified the smallest known binary star system to date. Called HM Cancri, its consists of two dead stars that revolve around each other in 5.4 minutes, by far the shortest known orbital period of any pair of stars.

The team, led by Gijs Roelofs of the Harvard-Smithsonian Center of Astrophysics, used the 10-meter Keck I telescope in Hawaii and its Low Resolution Imaging Spectrograph to study the velocity changes in the spectral lines in the light coming from HM Cancri.

They saw that as the stars orbited each other, the system’s spectral lines shifted periodically from blue to red and back, in accordance with the Doppler effect. With that velocity information, the astronomers were able to confirm the binary’s 5.4-minute period.

“When the first data from the Keck telescope arrived, and our quick analysis showed the periodic shift of the spectral lines, we knew that we had succeeded. More than ten years after its discovery, we finally had deciphered the nature of HM Cancri,” said Arne Rau of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the observations at Keck.

Astronomers proposed several years ago that HM Cancri was an interacting binary consisting of two dead stars and that the 5.4 minute period observed was indeed the orbital period.

The team had been trying to make precise velocity measurements to confirm the period since 2005.

X-ray evidence

HM Cancri was discovered in 1999 as a weak X-ray source in data from the German ROSAT satellite. It comprises two white dwarfs, burnt-out cinders of stars that were once similar to the Sun and contain a highly condensed form of helium, carbon and oxygen. In 2001, the X-ray, and also optical, data suggested that the two stars orbited each other in 5.4 minutes.

Another artist's conception of HM Cancri.

Another artist's conception of HM Cancri. One star is feeding the other.

But the information suggested that the binary system was roughly eight times the diameter of the Earth—equivalent to a quarter of the distance between the Earth and the Moon—or smaller. Astronomers were reluctant to accept this physical description without additional evidence. But at a distance of 16,000 light years from Earth, the binary system shines only one millionth as bright as the faintest stars visible to the naked eye, making it very hard to study. To determine with certainty the period of such a system, astronomers needed to use world’s largest telescopes to collect the additional evidence.

“This type of observation is really at the limit of what is currently possible. Not only does one need the biggest telescopes in the world, but they also have to be equipped with the best instruments available,” said team member Paul Groot of the Radboud University Nijmegen in the Netherlands.

As a result of the successful observations with Keck, astronomers now have a new cosmic laboratory to study the evolution of stars as well as general relativity.

“We know the system must have come from two normal stars that somehow spiralled together in two earlier episodes of mass transfer, but the physics of this process is very poorly understood,” said Gijs Nelemans of the Radboud University who was also part of the team.

He added that the system must be one of the most copious emitters of gravitational waves. “We hope to detect these distortions of space-time directly with the future LISA satellite. HM Cancri will now be a cornerstone system for the mission,” he said.

Adapted from information issued by Keck Observatory / NASA / Tod Strohmayer (GSFC) / Dana Berry (Chandra X-Ray Observatory) / Rob Hynes and Paul Groot, Radboud University.