Pluto was famously booted from the planet club due to a change in definition – now a new definition of a “planet” could be on the way. NASA/JHUAPL/SwRI –
Exactly what a planet is has been remarkably murky for centuries – what we now call planets, moons and even asteroids have all been lumped under that one umbrella at different times. But the real push for an official definition began in the 1990s, and gained steam in the early 2000s as a series of roughly Pluto-sized objects were discovered in the Kuiper belt, on the fringes of the solar system.
In 2006, the International Astronomical Union (IAU), the organization that officially names objects in space, finally established a definition. And it was almost deliberately designed to exclude Pluto – after all, if that’s a planet, then suddenly our solar system could technically have dozens, even hundreds of planets.
For an object to meet the IAU’s official definition of a planet, it needed to meet three criteria:
- it has to be in orbit around the Sun (that rules out all the moons)
- it has to have enough mass to become roughly round (there goes all the asteroids)
- it has to have cleared the neighborhood around its orbit (that rules out Pluto and everything else in the Kuiper belt)
But right from the beginning, this definition had its doubters. The third point, in particular, has been called “sloppy” by scientists.
“They didn’t say what they meant by clearing their orbit,” said Philip Metzger, a planetary scientist on the New Horizons mission, which sent a probe past Pluto in 2015. “If you take that literally, then there are no planets, because no planet clears its orbit.”
Another issue is that the current definition is very solar-system-centric, stating that a planet has to orbit the Sun. But in recent years astronomers have discovered well over 5,000 planets orbiting other stars. So, a team of scientists are proposing a new definition that not only accounts for this, but simplifies the criteria as well.
According to the new proposed definition, a planet is a celestial body that:
- orbits one or more stars, brown dwarfs or stellar remnants;
- is more massive than 1023 kg; and
- is less massive than 13 Jupiter masses (2.5 x 1028 kg)
This new definition expands on, clarifies and simplifies the existing one. Not only does it cover our own solar system and similar setups, but planets have been found orbiting multiple stars, as well as brown dwarfs (or failed stars) and stellar remnants like white dwarfs and neutron stars.
Having a lower limit of 1023 kg works because, the team says, it’s at that point that the gravity pulls the object into a spherical shape. We can’t usually see a distant exoplanet’s exact shape, but we can measure its mass.
However, those hoping Pluto might get to become a card-carrying planet again will be disappointed – its mass is just shy of the cut-off, while Mercury manages to scrape through.
The other advantage of this lower limit is that it puts a more direct number on what the original “cleared its neighborhood” clause was trying to achieve. Beyond this minimum mass, an object becomes “dynamically dominant,” either scooping up or ejecting smaller objects in its vicinity.
“All the planets in our solar system are dynamically dominant, but other objects – including dwarf planets like Pluto, and asteroids – are not,” said Jean-Luc Margot, lead author of the study. “So this property can be included in the definition of planet.”
And finally, the new definition places an upper limit on how big a planet can be. Some planets are so huge that their gravity triggers the thermonuclear fusion of deuterium in their cores, making them a kind of middle-ground between planets and stars. This is generally thought to begin in objects with masses of more than 13 Jupiters.
“Having definitions anchored to the most easily measurable quantity – mass – removes arguments about whether or not a specific object meets the criterion,” said Brett Gladman, co-author of the study. “This is a weakness of the current definition.”
The researchers plan to officially propose the new definition at the IAU General Assembly this August. The proposal is due to be published in the Planetary Science Journal.
Source: UCLA
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Yet Pluto is spherical, so 10^23 kg is not really the lower limit at which gravity pulls an object into a spherical shape. Why 10^23 kg (which Pluto is just shy of) if it’s not the lower limit for a spherical object? Almost seems like they are setting the bar at just the right point to exclude Pluto.