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Planet Friendships in Solar Systems Beyond Our Own

Exoplanet detection techniques improved in hopes of finding the expected co-orbiting planets

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Our understanding of astrophysics states that we should be seeing co-orbiting planets when astronomers look through their telescopes. Co-orbiting planets are planets of roughly similar size that have overlapping orbits around their host star. These two planets orbit so closely to each other that they influence the orbital paths and speeds of one another.

When solar systems and their planets are forming, scientists expect this formation to cause co-orbiting planets. For years now, astronomers have been cataloging exoplanets, planets in other solar systems, and have not observed any co-orbital planets.

Many of these distant stars are quite far away, so they are difficult to see even with humankind’s strongest telescopes. The planets that orbit these distant stars are even harder to spot. Astronomers are utilizing a variety of techniques to attempt to detect these planets, including the radial velocity technique and the star-transit technique.

The radial velocity technique tries to spot a planet going around a star by the subtle change in color of the star. When the planet is going away from Earth, astronomers can see the edge of the star as slightly redder, then at regular intervals later, they see the other edge of the star slightly bluer. These color fluctuations show astronomers that there is some planet going quickly around the star.

The star transit technique relies on a planet going in front of its host star, blocking part of the light from the star, like in an eclipse. Because some of the light is blocked, this makes the star look slightly dimmer to the telescopes. The planets orbit in standard patterns; so astronomers can see when the star light keeps dimming at regular intervals. From this information, they can determine that there is something orbiting around the star, like a planet. 

Despite these clever techniques, astronomers still have not found any instances of co-orbiting planets. These techniques for detecting exoplanets are clever but they are still only good at detecting large exoplanets. These techniques tend to favor the discovery of Jupiter sized planets and are less likely to find as many Earth sized planets. It’s simply easier for astronomers to see bigger, brighter celestial objects.

These researchers sought to determine exactly why astronomers detection techniques were not good enough to detect co-orbital planets. They also wanted to see what improvements they could make on the techniques to increase the likelihood of detecting this elusive phenomenon.

Like the diagram above suggests, the easiest way to see co-orbiting planets would be to see the entire orbital path from above. This would allow astronomers to see the crossing orbital paths, and notice how much the co-orbiting planets affect one another. Unfortunately, none of the current exoplanet detection techniques are able to notice planet’s orbital paths from the top-down view. Thus, the researchers had to determine what they could use from the radial velocity and star transit observations.

Using their expertise in astrophysics on how the planets should move in their non-standard orbits, the researchers analysed how the the star light should dim when a co-orbiting planet pair orbited in front of the star. If two planets are in a co-orbit, but only one ever crosses in front of the star, then this situation makes the co-orbit undetectable. However, if the two planets were to transition together in front of their host star, then co-orbiting planets will create a secondary fluttering pattern, or oscillation in the dimming pattern. With precise enough telescope, such a transition could be detectable!

Secondly, the scientists investigated how co-orbiting planets could be visible from their radial velocities. The scientists found that as long as the two planets’ orbital paths are closer to a circle (rather than an long oval) then the effects on radial velocity could be predicted. Circular, co-orbiting planets will have small variations, or perturbations, in their color as seen from Earth. These small variations can tell astronomers that they are seeing a co-orbital planets distorting each other’s orbit, rather than a single planet going around a star.

The scientists’ expansions on existing techniques are also useful to roughly determine the mass of the two co-orbiting bodies as compared to each other. This is exciting because in our search for life, astronomers and astrobiologists are often looking for other Earth-like planets, and part of being Earth-like is being similar in size. Furthermore, the scientists’ contribution to the field allows astronomers to look more closely at their existing data in the pursuit of co-orbiting planets. The search for co-orbiting planets is important for not only verifying astrophysical theories by observing them in nature, but these sort of detailed techniques allow us to reuse our existing telescopes and data. When we are clever with the data and instruments humankind already has, then we can extend the lifetimes and usefulness of our telescopes – all in the pursuit of the answer to the simple question, “Are we alone?”

Study Information

Original study: Detection of co-orbital planets by combining transit and radial-velocity measurements

Study was published on: March 7, 2017

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This summary was edited by: Gina Riggio