The discovery of 2003 UB313, the 10th planetThe newest size measurement comes from the Hubble Space Telescope. While for most telescopes the planet is too small to be seen as anything other than a dot of light, HST can (just barely) directly measure how big across it is. The measurement is extremely hard, however, even for HST, because even HST distorts light a little bit as it goes through the telescope, and we needed to be sure that we were measuring the actual size of the planet, rather than being fooled by distortion. So we waited until the planet was very close to a star and then snapped a series of 28 pictures and carefully went back and forth comparing the star and the planet. In the end, we determined that the planet is 2400 +/- 100 km across.
by Mike Brown
April 2006
The best ever picture from the Hubble Space Telescope, as unimpressive as it is (since the planet is so so so so so far away) looks like this:
When we initially guessed how big the planet was, we thought it was likely a bit larger, because we guessed that it probably reflected the same amount of sunlight as Pluto (about 60%). But this new size measurement tells us that the planet reflects considerably more sunlight than Pluto (86 +/- 7%)!. For more on this see below on what the planet is made out of.
The new HST measurement makes it sound like the previous measurement was "wrong," but it was not! All measurements in science are subject to uncertainty, and the group from Bonn carefully stated what their uncertainty was, just as we have with the new measurement....
http://www.onlineconversion.com/length_common.htm
2400 kilometer = 1491.29086137 mile
+/- 100 km, so...
2300 kilometer = 1429.153742146 mile
2500 kilometer = 1553.427980593 mile