One Planet, Two Stars: A System More Common Than Previously Thought

There are few environments more hostile than a planet circling two stars. Powerful tidal forces from the stars can easily destroy the rocky building blocks of planets or grind a newly formed planet to dust. But astronomers have spotted a handful of these hostile worlds.

A new study is even suggesting that these extreme systems exist in abundance, with roughly half of all exoplanets orbiting binary stars.

NASA’s crippled Kepler space telescope is arguably the world’s most successful planet hunter, despite the sudden end to its main mission last May. For nearly four years, Kepler continuously monitored 150,000 stars searching for tiny dips in their light when planets crossed in front of them.

As of today, astronomers have confirmed nearly 1,500 exoplanets using Kepler data alone. But Kepler’s database is immense. And according to the exoplanet archive there are over 7,000 “Kepler Objects of Interest,” dubbed KOIs, that might also be exoplanets.

There are a seeming endless number of questions waiting to be answered. But one stands out: how many exoplanets circle two stars? Binary stars have long been known to be commonplace — about half of the stars in the Milky Way are thought to exist in binary systems.

A team of astronomers, led by Elliott Horch from Southern Connecticut State University, has shown that stars with exoplanets are just as likely to have a binary companion. In other words, 40 to 50 percent of the host stars are actually binary stars.

“It’s interesting and exciting that exoplanet systems with stellar companions turn out to be much more common than was believed even just a few years ago,” said Horch in a news release.

The research team made use of the latest technology, speckle imaging, to take a second look at KOI stars and search for any companion stars. In using this technique, astronomers obtain rapid images of a small portion of the sky surrounding the star. They then combine the images using a complex set of algorithms, which yields a final picture with a resolution better than the Hubble Space Telescope.

Speckle imaging allows astronomers to detect companion stars that are up to 125 times fainter than the target, but only a small distance away (36,000 times smaller than the full Moon). For the majority of Kepler stars, this equates to finding a companion within 100 times the distance from the Sun to the Earth.

The team was surprised to find that roughly half of their targets had companion stars.

“An interesting consequence of this finding is that in the half of the exoplanet host stars that are binary we can not, in general, say which star in the system the planet actually orbits,” said coauthor Steve B. Howell from the NASA Ames Research Center.

The new findings, soon to be published in the Astrophysical Journal, further advance our need to understand these exotic systems and the harrowing environments they face.

2 Replies to “One Planet, Two Stars: A System More Common Than Previously Thought”

  1. At what threshold of sight, is viewing a companion star-system, too great to identify the second, or companion star?
    What are the larger orbits of companions? And is it a sure-thing that, our own sun, is all alone?

  2. What would present-day planetary scientists say about the comments relating to planets in binary systems in HABITABLE PLANETS FOR MAN, a book published by Stephen H. Dole, an aerospace techniques engineer from the Rand Corp., in the late Sixties?

    Chapter 4 says: “If the planet is in orbit around a binary star system, the stars must either be very close to each other or very separated so as not to interfere with the stability of the orbit of the planet and not to cause an intensity of the light that is too variable at the distance that the planet happens to be.” In Chapter 5 he returns to this matter and makes himself more clear: “As previously discussed, there can be habitable planets in binary star systems if the stars are so close to each other that there is only one ecosphere [now called the “Goldilocks zone”] around the pair of stars, or if they are so far apart that at least one of them can have an ecosphere that the other one will not interfere with.”

    On the next page it says: “Some authors have assumed that binary star systems, unless the two stars are are very far apart, cannot have stable planetary systems (Huang, 1959). However, in the three-body problem [certain matters suggest that] stable planetary orbits can exist within the ecospheres in most of the relevant binary systems, but the general conditions for the stability of the planetary orbits are difficult to calculate when the two stars are moving in eccentric orbits around their common center of mass./It follows that nearly all binary star systems (spectroscopic binaries) with stars that are close to each other could have habitable planets, and that in the visible ones very few have a distance between the stars such that they impede the existence of a normal ecosphere. However, (…).”

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