It may not be anything like Tatooine of Star Wars, but this discovery is still incredible. We've found a frozen, rocky planet orbiting one of its two parent binary stars in a stable Earth-like orbit. This significantly expands our sense of where life can emerge in the galaxy.
Binary star systems are quite common. So common, in fact, that they may account for as many as 50% of all stellar systems, though more conservative estimates place the figure at about a third. Regardless, the presence of so many binary stars likely imposes a profound constraint on galactic habitability; gravitational perturbations from a companion star can interfere in the formation and long-term stability of planets.
Prior to this study, all the planets discovered thus far in binary systems have been gas giants, including a binary system with more than one planet. It's possible, of course, that the moons of these exoplanets are habitable, but astronomers have no idea if rocky Earth-like planets in stable Earth-like orbits can form in these systems — though models suggest it is possible.
It now appears the simulations were right. Using a gravitational microlensing technique, an international team of astronomers led by Andrew Gould of Ohio State University has discovered a large terrestrial planet in a binary system.
An Entirely New Population of Exoplanets
Called OGLE-2013-BLG-0341LBb, the planet is located 3,000 light-years from Earth. It's about twice our planet's mass and is situated about 0.8 AU from its preferential host star. That's just slightly closer than Earth's distance to the Sun. But because its host star is only about 10% to 15% the mass of our Sun, it shines 400 times less brightly; this world is cold, dark, and likely uninhabited. At around 60 Kelvin (-352 degrees Fahrenheit or -213 Celsius), it's even colder than Jupiter's icy moon Europa.
But that's not the interesting part. What's exciting here is the planet's curious behavior. It orbits just one member of a pair of stars, practically ignoring the more massive companion star (spinning in what's called an "S-type" orbit). Planets that orbit both members of a stellar pair — called circumbinary planets — are quite common (they spin around in "P-type" orbits). Despite the close proximity of the two stars, OGLE-2013-BLG-0341LBb largely maintains its 0.8 AU distance, which is very good news for astrobiologists.
While this world is likely uninhabitable, its existence suggests the presence of an entirely new population of planets around these sorts of binary orientations — some of which may host planets that reside within the star's habitable zone. This study proves that terrestrial planets can form in orbits similar to Earth's — even in a binary system where the stars are not very far apart. In this case, the second star in the system is only 15 AU away — about the distance of Saturn from our Sun. But this binary companion is also very dim.
Importantly, however, binary star systems composed of dim stars like these are the most common type of star system in our galaxy. This discovery therefore suggests that there may be many more terrestrial planets out there that we thought— some potentially capable of harboring life.
The Power Of Microlensing
To make the discovery, Gould's team used a technique that's not often used to detect exoplanets. It's called gravitational microlensing, an optical phenomenon where light emanating from a distant object gets warped and distended by the gravity of another nearby object. This warping can create a magnification effect, making celestial objects, like planets and galaxies, easier to find.
Searching for planets within binary systems is tricky for most techniques because the light from the second star complicates the interpretation of the data. But in gravitational microlensing, astronomers don't even look at the light from the star-planet system. They just observe how its gravity affects light from a more distant, unrelated, star. Microlensing is thus offering astronomers a powerful new new tool to search for planets in binary star systems.
"Now we know that with gravitational microlensing, it's actually possible to infer the existence of a planet — and to know its mass, and its distance from a star — without directly detecting the dimming due to the planet," noted Gould in a statement. "We thought we could do that in principle, but now that we have empirical evidence, we can use this method to find planets in the future."
Read the entire study at Science: "A terrestrial planet in a ~1-AU orbit around one member of a ∼15-AU binary".
Image: Gould et. al/Ohio State University
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