Wednesday, June 27, 2007

Ever heard of of planet Eris?

This is planet Eris, in the outskirts of the Solar System, as seen through the eyes of the Hubble Space Telescope:

Hubble Telescope's Advanced Camera for Surveys has taken this image of Eris in December 2005. The analysis of several such photos yields a diameter of Eris of 2400±100 km, or 1500±60 miles. (Credits: HubbleSite News Release STScI-2006-16, M. Brown)

It's such a faint and blurred blob even in the Hubble Space Telescope because it is quite small, and far away from Earth: At the moment, Eris is close to its aphelion, at a distance of 97 astronomical units - that's 97 times the mean distance of the Earth from the Sun! You can explore its eccentric and highly inclined orbit with this applet from the JPL.

In fact, according to the new definition of the International Astronomical Union from last August, Eris is not a planet, but only a dwarf planet. However, it is larger than Pluto, and it has more mass than Pluto, as was reported in a beautiful short note in Science two weeks ago [Ref 5]! So, for all those who prefer to stick to the old definition of a planet, it may be the true ninth planet - unless some other, even larger guy shows up from out there in the Kuiper belt.

Eris was discovered in October 2003 by astronomers Michael Brown, Chad Trujillo, and David Rabinowitz [Ref 1]. It was given the provisional name 2003 UB313, or Xena for short. Soon after the discovery, its size could be measured by observations with a radio telescope of the Max-Planck Society [Ref 2] and the Hubble Space Telescope [Ref 3], and it came out that the new planet is larger than Pluto!

Moreover, there is a small moon in orbit around Eris, which was given the nickname Gabrielle. On September 14, 2006 the International Astronomical Union made official the names Eris for the planet and Dysnomia for its satellite.

A comparison of the sizes of (left to right) Eris, Pluto and Charon, the Moon, and the Earth. Eris' moon Dysnomia is not shown, but it is much smaller than Eris (diameter: 2400±100 km from HST, 3000±400 km from radio observation), Pluto (2300 km), Charon (1200 km), the Moon (3500 km), and the Earth (12800 km). (Credits: Max-Planck Gesellschaft Press Release News/SP/2006(10), Frank Bertoldi)

The discovery of Eris, and of several other large objects in orbits beyond Neptune, had spurred the hot debate about what is a planet, which then lead to the degradation of Pluto from planet to "dwarf planet" in August 2006. So it is fitting that Eris is the Greek goddess of strife and discord - Dysnomia, her daughter, is the goddess of lawlessness.

But in a time when ancient Greek gods and goddesses are reduced to large balls of rock and gas in space, even the goddess of lawlessness is subject to Newton's universal law of gravitational attraction. And this allows, in an elementary and elegant, classical way, to determine the mass of Eris.

The photo on the left is an image of Eris and its moon Dysnomia, the small spot left of Eris, taken with the Hubble Space Telescope on 30 August 2006. Dysnomia's projected orbit around Eris is superimposed on this photo in the image on the right (Credits: HubbleSite News Release STScI-2007-24, M. Brown)

In a first step, the orbit of Dysnomia around Eris has been determined from several observations with the Hubble Space Telescope and the Keck telescope in Hawaii.

The orbit of Dysnomia, the moon of Eris, as reconstructed from observations using the Keck telescope on 20, 21, 30, and 31 August 2006 and the Hubble Space Telescope on 3 December 2005 and 30 August 2006. Observations are show as crosses, the predicted positions at the time of observations are shown by circles. The solid circle in the center is 10 times the actual angular size of Eris. (Fig. 1 from Brown and Schaller, Science 316 1585 (2007 June 15), doi:10.1126/science.1139415. Reprinted with permission from AAAS.)

The observation of Dysnomia as shown in this figure reveals an apparent diameter of the orbit of roughly 1.1 arcsec, which, at the distance of 97 Astronomical units, or 97 × 150 million km ≈ 1.46·1010 km, corresponds to a diameter of 1.46·1010 × 1.1 × 2π / (360 × 3600) km ≈ 78000 km. A more detailed analysis yields an indeed circular orbit with a semimajor axis of r = 37500±200 km.

Now, this can be used to deduce the mass ME of Eris!

Using elementary Newtonian mechanics, we equal the gravitational attraction between Eris and Dysnomia at distance r with the centrifugal force,

GMEmD/r2 = mDrω2.

The frequency ω = 2π/T has to be calculated from the orbital period T of Dysnomia. With the equivalence principle at work, the mass of the satellite cancels out, and we can solve for the mass of Eris:

ME = r3ω2/G.

Now, the orbital period of Dysnomia has been determined quite precisely to 15.772±0.002 days. This yields ω = 2π/(15.77 d × 24 h/d × 3600 s/h) ≈ 4.63·10-6 s-1. Newtons constant is 6.673·10-11 m3 kg-1 s-2, and plugging everything together, the mass of Eris comes out as

ME = 1.7·1022 kg.

For comparison, Pluto has a mass of 1.3·1022 kg, the Moon of 7.35·1022 kg, and the Earth of 5.97·1024 kg. This means, the masses of Eris, Pluto, the Moon, and the Earth compare as 1.3 : 1 : 5.6 : 459.

And so, indeed, Eris and Pluto are on equal footing, revealed by the goddess of lawlessness and Newton's law!

Here are some "historical" references about Eris:

  1. Discovery of a Planetary-sized Object in the Scattered Kuiper Belt, by M.E. Brown, C.A. Trujillo, and D.L. Rabinowitz, The Astrophysical Journal 635 L97-L100 (2005 December 10) [PDF, ADS entry], arXiv:astro-ph/0508633 [see also: Eris webpage by Michael Brown]

  2. The trans-neptunian object UB313 is larger than Pluto, by F. Bertoldi, W. Altenhoff, A. Weiss, K.M. Menten and C. Thum, Nature 439 563-564 (2006 February 2) [PDF, ADS entry], doi:10.1038/nature04494 [see also: Press release by the Max-Planck Gesellschaft, comment by Frank Bertoldi]

  3. Direct Measurement of the Size of 2003 UB313 from the Hubble Space Telescope, by M.E. Brown, E.L. Schaller, H.G. Roe, D.L. Rabinowitz, and C.A. Trujillo, The Astrophysical Journal 643 L61-L63 (2006 May 20) [PDF, ADS entry] [see also: Press Release by the HubbleSite]

  4. Satellites of the Largest Kuiper Belt Objects by M.E. Brown et al., The Astrophysical Journal 639 L43-L46 (2006 March 1) [PDF, ADS entry], arXiv:astro-ph/0510029 [see also: Press Release by the Keck Observatory, News Item by Marcos van Dam, Dysnomia, the moon of Eris - webpage by Michael Brown]

  5. The Mass of Dwarf Planet Eris by Michael E. Brown and Emily L. Schaller, Science 316 1585 (2007 June 15), doi:10.1126/science.1139415 [see also: Press Release by the HubbleSite News item by the Planetary Society, Press Release by the Keck Observatory]

TAGS: , ,


  1. Great post, Stefan!

    While New Horizons is making its way to the Kuiper belt, another spacecraft is getting ready to fly to the Asteroid belt. These pictures show the last 2 months of Dawn being prepared for launch. It's almost ready to go now (launch window opens July 7). In these lastest pictures, you can see Dawn being unpacked, and then being hoisted way up the top of the tower to mate it with the Delta II launch rocket.

  2. Thanks for that nice post! I have to admit it's kind of funny to realize that what I've been told as kid (our solar system has nine planets Mein-Vater-Erklaert-Mir-Jeden-Sonntag-Unsere-Neun-Planeten) is wrong, and that the knowledge we will pass on to the next generation is more precise.

  3. Ceres [may already be?] and Sedna are also being considered for dwarf planet status.

    Planets are divided into rocky [4] and gas giants [4].

    Maybe all planets are not equal?

  4. Hi Stefan, interesting posting regarding our visual capacity for technical discovery. It really is amazing how technical knowledge will be advanced for near earth objects and such, how soon before we really have the tools to google-"other" Earths! paul.

  5. Goddesses of Lawlessness are important catalysts for change. ;-)

  6. Dear Amara,

    thank you for the hint at the DAWN mission, I had not heard of it before! The flight trajectory is a beautiful outward winding spiral - but it takes a painfully long time only to reach Ceres...

    That means, once New Horizons will have reached Pluto and Charon, all kinds of major bodies in the Solar System will have been visited by space probes? That's great :-)

    Best, stefan

  7. Dear Bee,

    yes, that's amazing how those "known facts" are changing! I mean, that's maybe similar for dark matter or dark energy, which wasn't around in the astronomy books when I was a kid - but somehow, these small planets far out there are much more tangible.

    Cheers, stefan

  8. Hi doug,

    Maybe all planets are not equal?

    maybe they all have their own personalities, as diverse as the Greek gods and goddesses that gave them their names ;-)

    Hi Paul,

    ... regarding our visual capacity for technical discovery.

    Yes, the possibilities of today's telescopes are really amazing. I have some volumes of a TIME-LIFE series ("Voyage through the Universe") about astronomy from around 1990 - just predating the launch of the Hubble telescope and large telescopes such as Keck - most photos in these books look pretty much outdated and quite poor compared to what we expect today!

    Best, stefan

  9. Hi doug,

    Ceres [may already be?] and Sedna are also being considered for dwarf planet status.

    I just found this page about the IAU resolution concerning the definition of a planet from last August. There, they count among the new category of "dwarf planets": Ceres, Pluto, and "2003 UB313", that is, Eris. I am not sure about the status of Sedna.

    Anyway, here is a news item from Science of two weeks ago, reporting about the Eris mass determination, with the nice title "Pluto's Bad Year Continues". It cites Frank Bertoldi, one of the radioastronomers of the University of Bonn in Germany who made the Eris size determination from the radio brightness, that he expects a few more dwarf planets to be found, which may be even larger than Eris.

    So the books will continue to be rewritten ;-)

    Best, stefan

  10. Dear Stefan, I will write up something this week (and tell you) about planet formation theories comparing/contrasting what is known about Ceres and Vesta in preparation for the Dawn launch because I know people are interested and there will be a lot of publicity (press thumbs for launch).

    Unfortunately, there seems to be a mistake with the Dawn photos at the link I gave previously.. now the Phoenix mission photos are appearing, when it should be only Dawn. Phoenix is on the adjacent launch pad to Dawn's and they are somewhat competing for the same launch window, so it's understandable to have photos mixed up. But be aware that there are Phoenix photos now in that collection too.

  11. Dear Stefan,

    Here you see pictures of the task of encapsulating the Dawn spacecraft to protect it during launch and ascent by giving it an aerodynamically smooth nose cone via a 'fairing'.

    The 'go' for launch is pending a final review of the weather and availability of support (airplane and ship) for post-launch tracking. The decision will be made Thursday, as I understand. The launch window opens up on Saturday, for five days. If Dawn misses it, then the launch will be pushed back to September. (Partly because of competition with the launch window of the Mars Phoenix spacecraft, getting ready now on the adjacent launch pad.)

  12. Dear Amara,

    thank you for the update!

    BTW, did you see, there is a report about Dawn in the New York Times: "Ceres, Now a Dwarf Planet, Is Scheduled for Exploration". They even write about the different compositions of Vesta and Ceres.

    How does this relate to theories about planet formation?

    Best, stefan

  13. "How does this relate to theories about planet formation?"

    Dear Stefan: The answer to your question. :-)

  14. Dear Stefan: The answer to your question. :-)

    Ah, thank you ... that's an impressive and informative post about Dawn and planet formation! And you're also a blogger now, congratulation :-)

    Best, stefan

  15. Dear Stefan, Thank you.. I am hoping to fill a niche for what I saw was a lack of good information on the Web for why some scientists think Vesta and Ceres are so interesting.

    I don't want to consider myself a blogger blogging days will be too irregular and infrequent for that

    Here is a reference that you might like too. I think that it is an excellent overview of what we know today for the formation of the solar system.

    Montmerle, Thierry; Augereau, Jean-Charles; Chaussidon, Marc; Gounelle, Mathieu; Marty, Bernard; Morbidelli, Alessandro (2006): "Solar System Formation and Early Evolution: the First 100 Million Years" Earth, Moon and Planets 37

  16. Dear Amara,

    thank you for the reference, that looks interesting!

    By some funny coincidence, I came across that issue of Earth, Moon and Planets a few days ago - it features several interesting review articles about the Emergence of Life, all available for free.

    Looking forward to read some interesting posts by you from time to time,

    best, stefan

  17. i need some information about this in albanain.*shqip*
    can someone tell me in some words ?


COMMENTS ON THIS BLOG ARE PERMANENTLY CLOSED. You can join the discussion on Patreon.

Note: Only a member of this blog may post a comment.