Kepler finds a forgotten Planet in a Triple-Star system

How mind-blowingly awesome it would be to have three suns to orbit around instead of one? This new planet called KOI-5Ab, (Yes! it needs a better name) has exactly that and much more. Though triple star systems are not so rare, this discovery deserves a special place, especially because this planet has a skewed orbit.

What did they discover?

The NASA Kepler mission at the beginning of its operations in 2009 spotted a planet about half the size of Saturn in a multi-star system. KOI-5Ab was the second planet detection from the mission, and exciting as it was they ultimately set it aside as Kepler went on discovering thousands of planets.

By 2018, it had discovered an impressive number of 2,394 exoplanets, and an additional 2,366 exoplanet candidates that need confirmation.

KOI-5Ab got mostly forgotten because it was complicated and Kepler had found many other easy pickings to research. But advanced observations from the second planet-hunting mission TESS (Transiting Exoplanet Survey Satellite) and many ground-based telescopes, have researched KOI-5Ab and confirm its existence. There are some fascinating details about this planet to ponder.

KOI-5Ab is unique because it orbits a triple star system, circling on a level, out of alignment with at least one star. It makes us wonder how everything in this system formed out of the same swirling clouds of gas and dust.


Quote from David Ciardi, chief scientist of NASA's Exoplanet Science Institute about finding the exoplanet KOI-5Ab

How did scientists confirm KOI-5B as a planet?

Data from the W. M. Keck Observatory in Hawaii, Caltech Palomar Observatory near San Diego, and Gemini North in Hawaii determined that KOI-5b planet is circling one of the triple-stars in the system. Still, they could not figure out if the signal was a mistaken glitch from one of the two other stars, or if the planet was real.

Then, in 2018, TESS came to help. Like Kepler, TESS seeks the flickerings of starlight which occurs when a planet passes in front of a star.

To confirm the elusive object is indeed a planet, scientists went back and re-investigated all the data, and then sought further cues from ground-based telescopes. Unlike TESS and Kepler observatories, scientists often use the Keck Observatory for follow-up searches of exoplanets. Keck measures the faint tremble in a star as a planet passes around it. Stars too experience the gravitational tug from the planets orbiting them as they move around.


The KOI-5 star system consists of three stars, labeled A, B, and C, in this diagram
The KOI-5 star system consists of three stars, labeled A, B, and C, in this diagram- Credits: Caltech/R. Hurt (Infrared Processing and Analysis Center, or IPAC)

An exoplanet collaboration group called the California Planet Search examined for any wobbles in Keck’s data on the KOI-5 system.

They detected a wobble generated in the inner companion star by the planet as it orbits the primary star. They confirmed that KOI-5b is indeed a planet that orbits its star roughly every five days.

Here are the orbital mechanics of the triple star system,

  • KOI-5Ab (planet) orbits Star A, which has a neighbouring companion, Star B.

  • The Stars A and B orbit each other every 30 years.

  • A third gravitationally captivated star, Star C, orbits stars A and B every 400 years


The triple-star system GW Orionis is a fascinating case. The gas around is torn apart and made shapeless due to the gravitational effect from its three stars. These effects might make planet formation rare in these types of star systems.

Here's a detailed post on the GW Orionis triple star system from Phil Plait for further reading: A TRIPLE-STAR GRAVITATIONAL DANCE CREATES RIDICULOUSLY COMPLEX SCULPTURES


A skewed orbit around the star:

The data also shows that the orbit of the planet seems unaligned with the Star B. Because of the bent orbit, it is hard to determine whether the whole star system formed from the same gas cloud. Astronomers are not sure what caused the misalignment of KOI-5Ab. But they think that Star B, kicked the planet during its development stage, disrupting its orbit and causing it to move inward.

Scientists believe modern tools, such as the Palomar Radial Velocity Instrument at the 200-inch Hale Telescope at Palomar, the NASA and National Science Foundation’s NEID instrument in southern Arizona and the Keck Planet Finder will expand our knowledge about exoplanets.


Source: Planetary Sleuthing Finds Triple-Star World

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