### How many planets are there?

While most people would answer that there are 9 or perhaps 10 planets, a proposal by the International Astronomical Union that will be voted on soon would significantly increase the number of objects that astronomers call planets. The proposal is to call any object that is large enough to make gravity cause it to become round a planet.

How many planets would this make? The nine planets that everyone knows are all round, so they are clearly planets. Ceres, the largest asteroid, is also round and would become a planet (the fifth). The big question, then, is how many new planets are there in the Kuiper belt, a region of rocky/icy bodies beyond Neptune, and the home of Pluto and 2003 UB313 ("the 10th planet").

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round. The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a planet.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects (for an explanation why, see the discussion on the size of  2003 UB313), but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of late August 2006,  44 objects this size or larger in the Kuiper belt (including, of course, 2003 UB313 and Pluto), and one (Sedna) in the region beyond the Kuiper belt. In addition our large ongoing Palomar survey has detected approximately 30 more objects of this size which are currently undergoing detailed study.

We have not yet completed our survey of the Kuiper belt. Our best estimate is that a complete survey of the Kuiper belt would more than triple this number.

For now, the number of known objects in the solar system which are likely to be round is 53, with the number jumping to 80 when the objects from our survey are announced, and to more than 200 when the Kuiper belt is fully surveyed.

The large number of new planets in the solar system are very different from the previous 9 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

### What does the new solar system look like?

Taking the number of planets from 9 (or 10) to 53 dramatically changes the look of the solar system. Here is the before (black circles) and after (add red ellipses) pictures:

(images may be freely used; credit: Mike Brown, Caltech)
In order from closest to furthest, the planets are now:

 name average distance from sun (semimajor axis, AU) estimated size (km) Mercury 0.39 4880 Venus 0.72 12,100 Earth 1.0 12,700 Mars 1.5 6780 Ceres 2.8 950 Jupiter 5.2 139,800 Saturn 9.6 116,500 Uranus 19.2 50,700 Neptune 30.0 49,200 2004TY364 38.72 540 2002KX14 39.01 560 2002XV93 39.22 430 2003VS2 39.27 610 1999TC36 39.27 440 2001QF298 39.30 490 Orcus 39.34 1100 2003AZ84 39.45 710 Pluto 39.53 2300 Ixion 39.65 980 Huya 39.76 480 2005RN43 41.53 740 1995SM55 41.64 470 2002MS4 41.90 740 2004SB60 41.97 560 2004GV9 42.23 680 2002UX25 42.53 810 Varuna 42.90 780 2002TX300 43.11 800 1996TO66 43.19 540 2003OP32 43.24 650 2003EL61 43.31 2000 Quaoar 43.58 1290 2003QW90 43.65 560 1999CD158 43.69 410 1997CS29 43.87 410 2000CN105 44.65 430 1998WH24 45.56 450 2005FY9 45.66 1600 2004PR107 45.75 520 2003MW12 45.94 740 2002CY248 46.18 410 2002KW14 47.08 510 2002AW197 47.30 940 2002WC19 47.67 410 2003QX113 49.56 450 2003FY128 49.77 430 2001UR163 51.40 620 2002TC302 55.02 710 1999DE9 55.72 490 2004XR190 57.36 540 2000YW134 57.77 430 2003UB313 67.69 2400 2005RM43 89.73 560 Sedna 486.0 1800

### Who has discovered planets?

Previously, the only official planet discoverers were William Herschel (one planet: Uranus),  Le Verrier/Adams (one planet: Neptune), and Tombaugh (one planet: Pluto).

Now the club is much much larger (some planets were discovered by teams which change a bit, in general the team name or the team leader is listed. Sometimes no discoverer is recorded):

• Brown et al.:  15 planets
• Deep Ecliptic Survey: 8 planets
• Near Earth Asteroid Tracking Survey: 4 planets
• Spacewatch: 4 planets
• Jewitt et al.: 3 planets
• Rupenstein, Ferrin, Danzl, Roe, Luu:  1 each