On Monday, March 15, astronomer Michael Brown and NASA colleagues announced the discovery of Sedna. (See: NASA press release, including photo panels.) The release notes that Sedna may be considered the first object discovered in the inner part of the Oort cloud of the solar system. Sedna's orbit also travels through the outer Kuiper Belt.
The New Scientist reports on Sedna, Distant object could be 'tenth planet'. To quote:
Pluto's title as the outermost planet could be in jeopardy, with the discovery of a large object orbiting the Sun far further out than any other. ...
Is this new world Sedna a planet or a planetessimal (more like an asteroid)?
In February, the team [that found Sedna] discovered an object about 70 per cent as wide as Pluto and in 2002 it found an object half Pluto's diameter, called Quaoar. The team estimates it will find as many as 10 new, large KBOs in the next few years, including a few objects larger than Pluto.
Sedna presents an awkward categorization challenge with there being no accepted definition of what is a planet: Sedna is considerably larger than the largest asteroid, Ceres, and it is not much smaller than the smallest planet, Pluto.
Sedna has a number of planet-like characteristics. While Sedna is far beyond Pluto, it has a stable though highly elliptical orbit in the planetary plane. (See Nature report: Astronomers spy new 'planet'. Also see: Graphic of location of Sedna in Kuiper belt.) The astronomers who discovered Sedna on Nov. 14, 2003, Michael Brown, Chad Trujillo, and David Rabinowitz, report that it may have a small moon. (See: NASA press release.) Another feature is that Sedna is the second reddest object in the solar system, after Mars.
At about 1700 km in diameter, perhaps a few hundred km smaller, Sedna approaches the size of other small planets. Yet, Brown, who co-discovered Sedna, does not consider Sedna nor even Pluto to be a planet.
For comparison, here are some diameters of smaller planet-sized objects in our solar system:
- Pluto: 2300 km
Two astronomers, John Stansberry and Gibor Basri, offer similar new definitions of planets that lower the size threshold. Their definitions contradict Brown and many other astronomers' position on the status of Pluto and now Sedna.
- Jupiter's four largest moons: over 3000 km
- Earth's moon: 3470 km
- Mercury: 4880 km
- Saturn's moon Titan: 5150 km
- Jupiter's moon Ganymede: 5270 km
- Mars: 6800 km
- Venus: 12100 km
- Earth: 12760 km
(Data are from JPL on Planets.)
The main point of the new definition is: a planet's lower limit is best defined as the size at which gravity of a planet pulls the planet's material roughly into a sphere. This planetary size is around 700 kilometers in diameter. (See longer definitions below.) This limit is larger than almost all asteroids, but not large moons or the newly discovered large Kuiper Belt Objects (KBOs).
Using this lower limit, our solar system now has at least 16 planets. These would be the 9 generally accepted planets, including Pluto (at approx. 2300 kilometers in diameter). The largest asteroid, Ceres, at 900 km in diameter, would qualify as a small planet. There are six recently discovered small planets in the Kuiper Belt.
The larger Kuiper Belt Objects, beyond the orbit of Neptune, have these approximate diameters:
- Sedna: 1700 km (NASA press release)
This list does not include Chiron, discovered in 1977, which is under 200 km in diameter and was promoted by some as a 10th planet when first discovered.
- 2004 DW: 1600 km, with size range of 840-1,880 km, depending on the albedo
- Quaoar: 1200 km
- Ixion: 1065 km
- Varuna: 900 km
- AW197: 900 km
(to be measured)
(On the above, see this link and click on this page title in left column: The 1000 km Scale KBOs .)
Currently, some astronomers consider Kuiper Belt Objects smaller than Pluto, and even Pluto itself, to be "planetessimals," akin to large asteroids.
One definition of planets focuses on the characteristics of orbiting around a star and gravitational effect on other planets. If so, what to we do with planets-sized objects floating in interstellar space? Is orbiting a star a necessary condition for being a planet?
These passages lay out a new definition of a planet:
In Is Pluto A Planet?, John A. Stansberry (1998) proposes:
A PLANET IS: any large, spherical, natural object which directly orbits a star, and does not generate heat by nuclear fusion. This definition is simple, is based on a physical definition of how large an object must be to be considered a planet, and without modification it will give the same results when applied to any planetary system. This definition distinguishes planets from asteroids and comets, which directly orbit our star, but are generally not large enough to be pulled into a spherical shape by their own gravity, and it distinguishes planets from stars, which frequently are in direct orbit around other stars, but generate heat internally by nuclear fusion. According to this definition, Pluto is clearly a planet.
Controversial Proposal Would Boost Solar System's Planet Tally to 12 (note that this planet count is already dated), offers details of some implications of a similar proposal, focusing on the lower limit of planet size:
Basri would like to accommodate Pluto and those who can't fathom its demotion. He proposes that the murky lower limit for planet-hood get set at a diameter of about 435 miles (700 kilometers). That's roughly the bulk needed to allow gravity to shape an object into a sphere, depending on density. ...
Note that the Kuiper Belt is very large: "The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies." (Source-see page with title: The Kuiper Belt and The Oort Cloud.) The distance of 100 AU equals about 15 billion kilometers. Sedna was found 13 billion kilometers out (at its nearest point). The Oort cloud, which includes most of Sedna's orbit, is vastly larger. The Oort cloud extends 3 light years from the Sun, or about 30 trillion kilometers.
More than 600 KBOs have been detected so far, but researchers extrapolate the limited sky surveys done so far to estimate there are about 100,000 of them bigger than 62-miles (100 kilometers). Enough larger KBOs exist to grow the solar system's planet count, based on Basri's definition, to two dozen within two years, according to estimates.
As the article above notes, some astronomers project that there are probably more Pluto-sized KBOs. The question now needs to be considered: how many large objects reside in the Oort cloud?
The definition of what is a planet will become more interesting if and when KBOs are found that are larger than Pluto and even larger than Mercury.
There is also a problem of definition with the upper limit of what is a planet. Presumably fusion can start in a very small star or brown dwarf at roughly 13 to 17 times the size of Jupiter. Where is this line to be drawn?
A few decades or centuries in the future, the view of what are planets likely will be different from our view today. The discovery of Sedna highlights the need for an agreed definition about what is a planet.