Super-sizing the 10th Planet again…

Thursday, February 2nd, 2006 at 11:05 AM by Administrator

Bertoldi's View of the Outer Solar System
(Click on image for full-size version.)
This is an illustration of the size of the world 2003 UB313 versus Pluto, it’s moon Charon, and the Earth and its Moon. The size of 2003 UB313 shown here is based on the estimate from Bertoldi and Altenhoff discussed in this article.

There is a general rule in science and that you should never trust a single observation. Earlier this week I reported a story that the “10th Planet” 2003 UB313 had been “downsized” with a new diameter estimate just a bit larger than Pluto’s diameter. Now come the millimeter-wave observations…

The Planetary Society’s website has a good story on the various recent estimates of the diameter of 2003 UB313. In a nutshell, we have three estimates that have hit the news:

  1. The initial estimate of the diameter of 2003 UB313 came from comparing its observed brightness and measured distance. This tells us how much light it is reflecting from the Sun. Then one has to estimate just how efficient 2003 UB313 is at reflecting sunlight. If it is very reflective, it can be a small world, if it is not reflective, its a large world. By assuming it was similar to Pluto in reflectivity (a.k.a. albedo), the team that discovered this world estimated its diameter as about 2700 kilometers (1620 miles).
  2. A few days ago it was reported that 2003 UB313 had been observed with the Hubble Space Telescope and it had been resolved into an image a “several” pixels across, implying a size just a bit larger than Pluto, about 2300 kilometers (1430 miles). This also implied 2003 UB313 was much more reflective than most (but not all) worlds in the solar system. Of course, if the image is only “several” pixels across and the report is a third-hand report of what someone hear at a public lecture, so this estimate should have been considered preliminary. In fact, as noted by the Planetary Society’s report:

    [T]his report was premature, and Brown has since stated, “Contrary to rumors otherwise, we’re just in the preliminary stages of analyzing the HST data. When we are done we should have a very precise measurement. I hope that we will have the HST analysis done within perhaps a month, and I’ll be able to say more then.

  3. Today we have a team of German astronomers led by Drs. Frank Bertoldi and Wilhelm Altenhoff (both from the Max Planck Institute for Radio Astronomy) have announced in a letter written to the journal Nature that they have made millimeter-wave observations of 2003 UB313 suggesting it’s diameter is considerably larger than Pluto’s.
    Albedo versus Diameter
    (Click on image for full-size version.)
    Albedo versus diameter for optical observations (blue) and millimeter-wave observations (red) of 2003 UB313. Where these cross gives the best estimate for the diameter of 2003 UB313.

    They made this estimate by looking at the 1.2 millimeter wavelength radiation given off by this world using the 30-meter IRAM radio telescope in Spain. Since the optical brightness of a world depends on how much sunlight it reflects, its optical brightness will depends on both the size of the world and its albedo (a.k.a. reflectivity). This leads to a possible set of size estimates illustrated by the blue line in the figure to the right. The millimeter-wave brightness of a world depends on its temperature and is only weakly dependent on its albedo. Millimeter-wave observations of 2003 UB313 lead to a possible range in sizes given by the red light in the figure. Where these two lines cross gives the best estimate for the size of Pluto since it is the one diameter consistent with both optical and millimeter-wave observations. Where the lines cross corresponds to a diameter of around 3000 kilometers (1860 miles). This large diameter also leads to a new estimate for the albedo of 2003 UB313 of around 60%, which is almost exactly the same as Pluto’s albedo.

I’ll finish this entry with a quote from Bertoldi’s news release quoting Mike Brown, the discoverer of 2003 UB313 and the person responsible for the Hubble observations reporting the “small” diameter estimate earlier this week about these new results:

The study that is coming out in Nature is the best info that we have for now about how big and reflective it is. The uncertainties are large, but it seems a solid result to me. I hope that we will have the HST analysis done within perhaps a month, and I’ll be able to say more then.

Related Articles:

  • Press Release: This press release from Bertoldi’s website contains more details than available from the online version of the Nature letter.
  • Nature News Article: Another good article on this large estimated diameter for the 10th planet from the Nature website itself.

Posted in Astronomy News

Comments are closed.