Thursday, July 07, 2016

My Three Suns: Our First Look at a Triple-Star System

2016 has been an exhilarating year for space enthusiasts, and we’re only in July. Actually, this is an exhilarating year for anyone interested in where we came from and what else is out there. So far we’ve seen the first (and second) detection of gravitational waves, a rapidly expanding list of exoplanets, and Juno’s successful arrival at Jupiter’s doorstep, to name a few highlights. Today in the journal Science, astronomers announced another crazy milestone, the first image of a planet with three suns.

This image shows the newly discovered planet in the triple-star system. The picture was created from two separate SPHERE observations: one to image the three stars and one to detect the faint planet. The planet appears vastly brighter in this image than it would in reality, compared to the stars.
Image Credit: ESO/K. Wagner et al.
It sounds like science fiction, but it’s all gravity. When we talk about the Earth orbiting the Sun, most of us picture the Earth tracing out a path around the Sun while the Sun stands still. In reality, both the Sun and the Earth orbit the same point, the center of mass of the Sun-Earth system. In our case, the Sun is just so much more massive than the Earth that the center of mass and the center of the Sun are practically the same thing.

With this in mind, here is the most likely scenario: The planet orbits around a bright, heavy star. Two fainter stars are very close to each other and orbit one another. In addition, the bright star and the two faint stars are locked in an orbit around the center of mass of the triple-star system.

If this is the case, the planet is sometimes far off to one side of the system and at other times right between the bright star (its main sun) and the other two stars. The researchers estimate that for about half of the planet’s journey around its main sun, which takes around 550 years, an inhabitant looking out from his or her front porch would see three suns! Keep in mind that this is the most likely scenario—others are possible but we need more follow-up observations to know for sure.

This graphic shows the orbit of the planet (red line) and the orbits of the stars (blue lines).
Image Credit: ESO
The planet was discovered by Kevin Wagner, a PhD student at the University of Arizona, using images from the European Southern Observatory’s Very Large Telescope (VLT) in Chile. The VLT consists of four ground-based telescopes that work together in a way that provides astronomers with significantly more detail than the sum of its parts. This planet was the first one discovered with SPHERE, an instrument on one of the VLT telescopes that aims to directly image planets outside of our solar system.

Most planets outside of our solar system are detected indirectly, for example by measuring changes in the light reaching us from a star that are characteristic of planet passing in front of the star. SPHERE is more direct. Using sophisticated technology it looks at a star, filters out the light coming from the star, and produces a kind of infrared photograph of what is left. Planets give off a lot of infrared light. We can tell us quite a bit about what a planet is like by studying its infrared signal. The new planet is about four times bigger than Jupiter and approximately 1,000° F. That’s big and hot compared to our planet, but it’s actually one of the smallest and coldest planets we have imaged.

This isn’t the first triple-star system with a planet that astronomers have discovered, but we’re still in single digits. Earlier this year researchers at the Harvard-Smithsonian Center for Astrophysics announced a similar system with a stable Jupiter-like planet in its orbit. That was number four. Multi-star systems are just as common as single-star systems like ours, but it’s not clear yet how common it is that they have planets.

Compared to planets in other multiple-star systems, the newly discovered planet has a much wider orbit—meaning that it’s a lot farther from its main sun. This makes the discovery extra surprising. Scientists expect that planets in triple-star systems don’t stick around very long because as their distance from the stars changes, so does the strength of the radiation and gravity they experience. The changes are even more dramatic for planets in wider orbits. We don’t know yet how stable the orbit of this planet is, but the fact that we’ve seen it this early in the search suggests this situation may be more common or stable than previously thought. It is a good reminder that, as usual, we still have a lot more to learn!
This artist's impression shows a view of the triple-star system from close to the planet. The planet is currently known as 131399Ab and appears in the lower-left corner of the picture.
Image Credit: ESO/L. Calçada
Kendra Redmond

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