Pluto Gains Status Among the Dwarfs

The thing that triggered the unceremonious demotion of Pluto to dwarf planet by the IAU in 2006 was the discovery of several similar Trans-Neptunian objects, namely Makemake, Haumea and Eris. Eris created the biggest problem, because it was estimated to be slightly larger than Pluto. So was Eris the 10th planet? How many more planets were we liable to find? Or were these cold, barren worlds something else?

Rather than have a Solar System with 20 planets, it was decided to redefine "planets" and the sub-category of "dwarf planet' was created to account for this new growing number of smaller objects in the outer reached of the Suns influence.

This week Pluto has regained its status as king of the dwarfs. Eris has turned out to be smaller than Pluto after all.

November 5, a chance alignment provided new data. As Eris crawled along its orbit, some 14 billion km from Earth, it passed in front of a distant star from Earth's vantage point, casting a small shadow across our planet, an event known as an occultation. By timing the duration of the occultation at multiple sites, researchers can estimate the size of the shadow and hence the size of an object.

Combining results from several observatories able to observe the occultation yields a diameter that is "almost certainly" less than the 2340km of Pluto. However, this opens the door on another mystery.

Eris's mass, determined from the orbit of its moon Dysnomia, is about 25% greater than that of Pluto. The newly calculated diameter doesn't change that fact. So if these results hold true, the density of Eris is even higher than we thought. Its albedo (reflectivity) is greater than estimated before too, since it is reflecting light to us from a now reduced surface area. Not long ago this would have thrown serious doubt on the results, but earlier this year astronomers found that the large Kuiper Belt object 50000 Quaoar is essentially a dense rock. 

Obviously, these interesting new members of the solar system have plenty of secrets to reveal, and as we find more of them Pluto's status may again change in relation to them, but for today anyway, Pluto is again king of the Kuiper Belt. 

Hail, Pluto. Long live the King.

Venetia Burney, The Girl Who Named Pluto

Do you know where Pluto got its name? I didn't know until I stumbled on this quite by accident.

Venetia Burney was born July 11, 1918. On March 14, 1930, when she was only 11 years old, Venetia's grandfather had just read an article in The Times about the discovery of a new planet. Venetia recalled some years later what transpired over breakfast that day.

"It was about 8 o’clock and I was having breakfast with my mother and my grandfather, and my grandfather as usual opened the paper, the Times, and in it he read that a new planet had been discovered. He wondered what it should be called. We all wondered. And then I said, “Why not call it Pluto”. And the whole thing stemmed from that."

Her grandfather forwarded the suggestion to astronomer Herbert Hall Turner, who cabled his American colleagues at Lowell Observatory. Clyde Tombaugh, the discoverer of the new planet, liked the name because it started with the initials of Percival Lowell who had initiated the search for Pluto. On May 1, 1930, the name Pluto was formally adopted.

An interesting side note to this story is that her grandfather's brother, Henry Madan, was the person who suggested the names Phobos and Deimos for the moons of Mars in 1878.

She lived long enough to see Pluto demoted to dwarf planet status, and commented in an interview while the debate about the definition of a planet raged on, "At my age, I've been largely indifferent to the debate; though I suppose I would prefer it to remain a planet." She died on April 30,  2009.

The asteroid 6235 Burney was named for her, and The Student Dust Counter, an instrument on board the New Horizons spacecraft, is also named after her.

Now there is a student competition called Naming X that challenges students to come up with the name of the next planet to be discovered. The deadline for submissions is April 15, 2010 and winners will be announced April 30.

My suggestion for the next planet, or dwarf planet, is Erebus, who was the son of the Greek god, Chaos, and represents the personification of darkness and shadow. You can steal that and enter it in the contest if you like, with my compliments.

This collage shows the relative sizes of the original three dwarf planets (Pluto, Ceres, and Eris) as compared to Earth. It also shows Pluto's large moon Charon (and its two small moons Nix and Hydra) and Eris's moon Dysnomia to scale.

Images courtesy of NASA, ESA, JPL, and A. Feild (STScI).

Plutoids

It seems the IAU is trying to make the public a little happier about demoting Pluto from planetary status. They have invented and adopted a new term to describe Pluto and its brethren, "Plutoids".

According to the new definition, plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made.

The saddest part of this news really affects Ceres, formerly an asteroid, a minor planet and now dwarf planet. Even though it is very similar to Pluto and Eris, it does not fit the definition of a plutoid because it is located in the asteroid belt between Mars and Jupiter. Not only that, but since we don't believe any more Ceres sized objects exist inside the orbit of Neptune (presumably, we would have already discovered them) there are no plans to propose a new name for these type objects.

So, we have adopted a new term for objects we believe will be commonplace one day, but have no special term for the one and only dwarf planet that has survived billions of years in the asteroid belt. Poor Ceres. I guess no one takes you cereously...