What is a planet? Despite decades of trying, astronomers still haven鈥檛 found a definition that everyone can agree on 鈥� and the problem only gets harder when considering worlds outside our solar system. The latest attempt to define an exoplanet has sparked fresh controversy.
The International Astronomical Union (IAU), arbiter of all things cosmic, set out a final definition for planets in our solar system in 2006, famously demoting Pluto to a dwarf planet in the process. Its first requirement is that a planet is in orbit around the sun, which rules out all exoplanets, meaning these objects need their own definition.
An IAU definition of exoplanets set out in 2003 included any object below 13 times the mass of Jupiter, a cut-off chosen because objects of this mass with the same chemical composition as the sun start undergoing fusion of deuterium, a form of hydrogen. At the other end of the scale, it excluded anything below the minimum size of a planet in our solar system, although this in turn wasn鈥檛 actually well defined.
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鈥淪ince that time, we have discovered many exoplanets and many different systems,鈥� says the IAU鈥檚 鈥� NASA puts the latest count at . 鈥淭he knowledge we have about these exoplanets is totally different from the knowledge we had in 2003.鈥�
To better conform to the new discoveries, the IAU鈥檚 working group on exoplanets, a body of more than 400 astronomers, voted in 2018 for a definition that adds a new requirement: an exoplanet鈥檚 mass must be less than 1/25th of the mass of the object it orbits.
This mass ratio is important because it implies something about a planet鈥檚 formation process, says at the University of Edinburgh, UK. 鈥淚f the mass ratio is fairly large, this is something that has formed in a disk around its star, like a planet, as opposed to something that forms more like a binary star.鈥�
The new definition also decreed that an exoplanet must be 鈥渃learing the neighbourhood鈥� in its orbit, meaning that it has gravitationally removed other objects of similar size. This brings exoplanets in line with the definition of a planet within our solar system, and it is that last requirement which Pluto fell foul of years earlier, thanks to its large moon Charon.
Although this was voted on in 2018, details have only now come to light with a newly-published explanation of the decision that has caught the attention of the wider astronomical community, and not everyone is happy.
at the University of Herefordshire, UK, called it 鈥溾�� in a tweet, saying it would be impossible for astronomers to detect whether an exoplanet has cleared its neighbourhood, while the mass ratio requirement means free-floating 鈥�rogue planets鈥� are technically not planets because they don鈥檛 orbit another body. Also, other objects that orbit brown dwarfs, which are massive objects that aren鈥檛 quite stars, would now count as planets.
The mass ratio also kicks some existing exoplanets out of the club. 鈥淪everal objects that are listed in the NASA Exoplanet Archive are no longer technically exoplanets,鈥� says at Columbia University in New York, such as MOA-2010-BLG-073L b, which is 11 times the mass of Jupiter. 鈥淭his is because they orbit very low mass stars, and so dividing that by 25 ends up cutting out [such] super-Jupiters.鈥�
Astronomers understand that definitions change as more information comes in, but it is in the marginal cases, where things can be considered a planet or a star, that questions often arise. 鈥淚f you have a 13 Jupiter-mass object orbiting a star, and then you have a 12.5 Jupiter-mass object orbiting a star, and one is just slightly massive enough that it has deuterium fusion, then is that enough to call one a planet and one not?鈥� says Biller.
Although the debate will continue, it may not have much impact. 鈥淪cientifically, it doesn鈥檛 matter too much in my opinion,鈥� says Kipping. 鈥淎 rose by any other name still smells as sweet. Nature has no need to subscribe to our desire to neatly categorise things into different boxes.鈥�
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