Mass of dwarf planet Eris 27% greater than Pluto
Artist's concept of Eris. Photo:
NASA/JPL-Caltech
.
See a larger version of this image.
In 2007, when astronomers measured the mass of the dwarf planet Eris, they found that it’s 27% more massive than Pluto.
Astronomer Mike Brown discovered Eris. He also recently found its tiny moon.
Mike Brown: We got lucky in that Eris has a moon going around it. And by tracking the orbit of the moon around the dwarf planet, you actually get to weigh how much the dwarf planet weighs.
Once they found this moon, astronomers were able to “weigh” Eris. The moon, called Dysnomia, zips around Eris about every 16 Earth-days. By tracking it, Brown determined Eris’ mass.
Mike Brown: And it turns out that it’s about 27% more massive than Pluto. And the uncertainty there is only about 2%. So, this was sort of the last chance that Pluto had, and now it’s definitely smaller and less massive.
When it was discovered in 2005, some thought Eris should be considered the 10th planet of our solar system. Everyone still considered Pluto a planet then. At first, Eris was thought to be slightly larger than Pluto. Now – with the help of Eris’ moon – Eris is known to be 27% more massive than Pluto. If Pluto had remained a planet to the entire community of astronomers, surely Eris would be considered the 10th planet.
In 2006, the International Astronomical Union (IAU) voted to demote Pluto as a full-fledged planet. The IAU now considers both Eris and Pluto to be dwarf planets. And yet there are many similarities between our own Earth and moon and the dwarf planet Eris and its moon.
Mike Brown: So the moon is in a very circular orbit around Eris, and we think that that’s telling us something important. We think that it actually formed much in the same way that the Earth and moon formed, such that some sort of large object hit both of these things in the distant past and made a big cloud of debris that coalesed to form the moon. We think the same thing happened for Eris and Dysnomia.
Thanks today to Research Corporation, America’s first foundation for science advancement.
Weighing Eris, Pluto’s heftier twin
Listen to a 12-minute podcast with Michael Brown, who discovered Eris.
This discovery changes nothing regarding Pluto’s status. Mike “Pluto is dead” Brown is wrong in his non sequitur argument when he says “this is the last chance that Pluto had.” Many in the astronomy community still consider Pluto a planet. There is hardly consensus otherwise, as Brown wants people to think. Eris
being larger does nothing to negate the argument that Pluto is the solar system’s ninth planet and Eris its tenth planet. Why the tenth planet cannot be larger than the ninth, in Brown’s view, is beyond understanding. How can the status of another object, rather than its own characteristics, define what Pluto is? The truth is astronomers remain very divided on this issue, and the debate is not over.
In his conversation with me Mike Brown admitted that he had a lot riding on Eris. Few people get a chance to discover a new planet! I invite readers here to check back later today or tomorrow for a podcast interview with Dr. Brown with his thoughts on the IAU decision.
I agree: this discovery changes nothing regarding Pluto’s planetary status. Pluto is still the ninth planet, and Eris is still the tenth planet.
I congratulate Brown and his colleagues on the original discovery of Eris, and now on the measurement of the planet’s mass; but it’s silly of him to say that this was the last chance Pluto had.
We’ve known since 2005 that Eris is larger than Pluto. Ever since then, most of us have guessed Eris was probably more massive, too. Now Eris is indeed confirmed as being more massive than Pluto. But so what? How does that change Pluto’s planetary status? It doesn’t. Pluto is a planet, and so is Eris.
Ok, so Pluto’s still a planet to you. Shouldn’t it be the 10th planet? Eris is bigger (and heavier), has just as regular an orbit, and is closer. If Pluto’s a planet, any description of Planet that’s not entirely arbitrary will give us 10, and Pluto, as the farthest, should be the tenth.
Actually, Pluto is not farther than Eris. Ken Croswell’s book Ten Worlds gives the following data on Pluto and Eris:
Pluto: Mean distance from the Sun: 3,670,000,000 miles
Eris: Mean distance from the Sun: 6,310,000,000 miles
In kilometers:
Pluto: 5,910,000,000 kilometers
Eris: 10,200,000,000 kilometers
And in astronomical units:
Pluto: 39.5 astronomical units
Eris: 67.9 astronomical units
According to page 44 of Ten Worlds, Eris is “the farthest thing yet seen in the solar system.” Because of its elliptical orbit, Eris is currently about 97 astronomical units from the Sun.
So: Pluto is the ninth planet; Eris is the tenth planet.
Here are the ten planets in order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and Eris.
Here are the ten planets in order of diameter: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, Eris, and Pluto.
And here are the ten planets in order of mass (note the switcheroo between Uranus and Neptune): Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury, Eris, and Pluto.
Why is the acceptance of “change” so difficult for so many of you when it deals with more fully understanding the structure and configuration of our universe?
As far back as Intermediate School (and we’re talking “decades” ago) I was aware of the controversy over whether or not Pluto should have been classified as a “true” planet in the sense that the astronomical community came to understand the concept, and after having considered the arguments and weighing the evidences I was well prepared even way back then to accept Pluto’s demotion from our solar system’s “planetary” status.
“Pluto” is a “planet” in the most “technical” sense, in that it revolves around our sun on a similar “plane,” which is from where the word “planet” is derived… but there are COUNTLESS celestial objects which similarly travel along the same “plane,” yet we do not go out of our way to consider them “planets” in the “classic” sense.
Admittedly, I took a great deal of interest following the discoveries of 2003 VB12 (“Sedna”) and UB313 (“Xena”/Eris) regarding speculations that one or the other might have been given “10th Planet” status, which would have pretty much ended the debate and secured “Pluto’s” place as our solar system’s 9th planet… but it was not to be; the astronomical community had made its decision, “Pluto” had been “demoted” to “dwarf planet” status, it seems fair on many levels, it opens the door for other new discoveries in our solar system to have a “specialized class” to which they can proudly belong, and above all I can live with it and proudly teach it to my children.
So “Pluto” is no longer a “planet”? It’s NOT that big a deal! Just enjoy your happy memories from when it was, accept the new reality, and don’t worry yourself sick over it.
The astronomical community has NOT made a decision concerning Pluto. Nearly as many astronomers signed the petition protesting the IAU vote as took part in the IAU vote.
Also, there are NOT countless objects the size of Pluto in our solar system. Pluto is the tenth largest known object that orbits the Sun.
Pluto is a planet. So is Eris.
I agree with Magenta. The IAU handled this very poorly.
It’ll be interesting to see what happens.
Most astronomers are NOT members of the IAU.
By the way, one MAJOR problem with the eight-planet proposal is that it applies different standards to objects of exactly the same mass and diameter, based on the idiotic demand that a planet “clear out its zone.”
Consider Mercury. Everyone considers it a planet.
But guess what? Suppose an object having exactly Mercury’s mass and diameter orbited the Sun in the Edgeworth-Kuiper belt. This “twin of Mercury” would NOT be classified as a planet under the IAU proposal! Why not? Because it hasn’t cleared out its zone!
Absurd! You have two identical objects going around the Sun, and one of them is classified a planet, and the other one is classified a nonplanet.
In contrast, if we define a planet of the Sun as a Sun-orbiting object whose mean diameter equals or exceeds Pluto’s, then this “twin of Mercury” WOULD be classified as a planet.
I too agree with Magenta. The astronomical community has NOT made a decision on this. A tiny minority of that community with its own agenda resorted to surreptitious means to impose that agenda. Thank goodness, enough people, including professional astronomers, see through this sham and won’t allow their vote to be the last word on this issue.
The issue here isn’t acceptance of change but whether a proposed change makes sense, is logical, and amounts to something more than one point of view in a controversy. The IAU decision does not create or reflect a new reality but imposes one point of view that is hardly shared by a majority of experts in the field.
Ideally, there should be a broad category of “planet” with various subcategories such as terrestrial planets, gas giant planets, Kuiper Belt planets, and minor planets (asteroids such as Ceres, etc.). All of them would fall under the umbrella of planets but would be distinguished further by their makeup and characteristics. If that means there end up being 100 planets in the solar system, so what? Limiting the number of planets just because it’s convenient to us is hardly a scientific argument.
Pluto and Eris are planets, and this is what I will proudly teach my nephews.
Actually, I do think limiting the number of planets to objects the size of Pluto and bigger is a very good idea. It’s important that the word “planet” retain the connotation of significance. If instead every last iceball is deemed a planet, then that connotation will no longer exist.
Pluto’s diameter is an excellent place to draw the line between planets and nonplanets. If we draw the line there, we have sufficiently few planets—ten—that “planet” refers to a significant body. Plus, we don’t have to cast out a world that has, since its discovery, been deemed a planet. The only change we have to make is to add Eris to the roster of planets in the solar system.
But history shows that happens every 75 years or so: Uranus in 1781, Neptune in 1846, Pluto in 1930, and Eris in 2005.
Magenta is patiently awaiting the year 2080!
It doesn’t matter if the planet ERIS is bigger than Pluto its still a dwarf planet
Why? What makes something a “dwarf planet?” The IAU’s definition is flawed to the point of being illogical in that it states that “dwarf planets” are not planets at all. This makes absolutely no sense. The requirement of “clearing its orbit” is also highly problematic, as the further a planet is from the sun, the harder it will be for that planet to clear its orbit. If Mercury were in Pluto’s orbit, it would not “clear its orbit” either. Adding this requirement was simply the agenda of a small group of dynamicists at the IAU conference and does not even reflect the views of most planetary scientists.
i was just passing thrugh and seen the site by chance i have to tell you i remember when its was just the 9 planets now as i see we have found so many more that are so huge!!! wow just to think that some day we may find one that will be just like earth but only the size antares ! some day … some day bless them who have brought the outer world to us to me it means the exploreation of new minerals that might be thare ?? maybe we can only imagen