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KOI-5Ab is a newly discovered planet in a triple-star system. It is a great example of the kind of astonishing discoveries that result from co-operation between large teams of astronomers using different types of telescopes and observation techniques.

Major discoveries these days, particularly in the fields of astronomy and physics, are increasingly achieved by teams of dozens or even hundreds of scientists combining data from multiple experiments and observation techniques.

 Trying to observe an exoplanet orbiting around a distant star is a bit like trying to see a firefly crawling on a searchlight, so the vast majority of exoplanets have been discovered using a variety of clever indirect techniques.

One of these is the radial velocity technique, which has been used to discover 833 exoplanets so far. This technique measures tiny shifts in the color of light from the star as it is gently tugged by its orbiting exoplanet.

Most of the early exoplanet discoveries were made using this technique.

Radial velocity was first, but now more than three-quarters of the known exoplanets have been discovered using the transit technique. This technique works by measuring a star's brightness over time, watching for regularly repeated drops in brightness, which could be caused by a planet passing in front of a star during its orbit.

The downside to the simple transit technique is that there are other astrophysical effects that can cause the same periodic drop in brightness, like background stars that vary in brightness, or starspots (like sunspots). Because of this, when interesting signals are first discovered by transit surveys, they are dispassionately numbered as "objects of interest" until they are validated as real exoplanets by another exoplanet detection technique, often radial velocity.

Some of the really remarkable exoplanet discoveries to date include planets that orbit around a pair of stars  and seven exoplanets in the same system all closer to their star than Mercury is to our sun, evaporating planets and a brown dwarf with rings .

All of these discoveries required a lot of additional modeling and data collection in order to understand the systems, but one of the most complicated exoplanet systems yet was announced in January 2020.

Kepler Object of Interest 5 (KOI-5) was one of the first batch of possible exoplanets sent down by the Kepler space telescope in 2009. But the first follow-up data quickly showed the system was complicated by an additional star and weird follow-up observations. 

High-resolution imaging by one team of astronomers was combined with longer time baseline radial velocity data from another team and the story began to emerge: KOI-5 was a triple-star system with an exoplanet orbiting one of the stars. This discovery was presented at the January 2021 American Astronomical Society meeting, and a peer-reviewed paper is forthcoming.

Two sun-sized stars, designated A and B, orbit each other every 29 years in the middle of the system, while a third, smaller star orbits the two central stars every 400 years. The discovered planet is called KOI-5Ab, because it orbits star A, on an orbit that is tilted wildly away from the plane of the stars' orbits.

Data from Kepler and TESS, which required the effort of dozens of astronomers working together, has revealed the size of KOI-5Ab: seven times the radius of the Earth. Another team of astronomers used radial velocity data to measure the mass of KOI-5Ab: 57 times the mass of the Earth. Combining these numbers gives the density, and tells us this planet is a gas giant planet, a bit smaller and denser than Saturn.


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