Nowadays, more than 6,000 planets are known which, like ours, revolve around a star. On the other hand, only 18 planets have been found that orbit a binary system, with two stars. For this reason, they are considered a rarity. They are so rare that the best known of them all is Tatooine and, far from existing, it belongs to Star Wars fiction. However, a team of scientists from the University of New South Wales has decided to change the method we use to search for planets. In doing so, they have suddenly found 27 candidates for planets with two suns.
An underused method. The method that have used These scientists know as apsidal precession. It was already used in the past to characterize binary star systems. However, until now it had not been used to check if there is another object, such as a planet, within that same system.
Broadly speaking, it consists of locating possible changes in the eclipse calendar of the two stars. If these changes cannot be explained with general relativity or stellar physics, there must be something disturbing their movements. This is how, thanks to the TESS telescope, 27 candidates for circumbinary planets (with two suns) have been located, although it will still be necessary to confirm which ones are really planets.
Stars playing hide and seek. Eclipses occur when, from the position of the telescope observing them, one star interrupts the light of the other. In a known binary system, these eclipses are predictable. Therefore, if we see something that doesn’t add up, there could be a planet in the way. TESS typically relies on the transit method to detect exoplanets.
It detects periodic disturbances in a star’s brightness, which could indicate that a planet is crossing between it and the telescope. However, if the planet has an irregular orbit that is not in the telescope’s line of sight, it may go unnoticed. However, with this new method that is not a problem, because you do not have to see the planet or the changes in the brightness of the star. It focuses rather on the gravitational effects that affect its two stars. It doesn’t matter that its orbit is not visible to us.
It’s just the beginning. This team has detected 27 candidates for circumbinary planets in a total of 1,590 two-star systems analyzed. That means about 2% of these systems could host planets. If this is true, thousands more planets could soon be detected. For a long time, exoplanet detection would have been highly biased.
Artist’s representation of a system with two stars
A great variety. The smallest possible circumbinary planet that has been detected has a mass similar to that of Neptune, while the largest is 10 times more massive than Jupiter. The closest one is 650 light years away from us, while the furthest one is 18,000 light years away. There are candidates in the northern and southern skies. In short, there is a great variety. That also supports the hypothesis that there are a wide variety of planets out there that, until now, have been invisible to us.
And now what? Now it will be necessary to check which of those 27 candidates are really planets. Some stars, such as brown dwarfs or white dwarfs, could also alter the eclipse calendar of the binary system. Even black holes could do it. Therefore, it must be ruled out that it is any of these phenomena. To resolve this question, another instrument will be used, the Anglo-Australian telescope from Coonabarabran. With it these other very massive objects could be detected. If no plausible explanation is found, it would be concluded that it is a planet.
What we can learn. Having techniques to detect another totally different type of exoplanet gives us much more information on planetary formation. There could be planets similar to Earth, whose only difference is the existence of two suns. Some of them could even host life or have hosted it in the past. The range of possibilities would open up greatly. About half of the stars in the universe are found in binary or multi-star systems. And all of that is still unexplored.
Image | Star Wars | NASA/JPL-Caltech/T. Pyle
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