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Dwarf planets have their day

Dwarf planets are big this month.

Writing for Space.com, Starry Night Education's Geoff Gaherty points out that two of the best-known dwarf planets, Pluto and Ceres, are in prime positions for viewing over the next couple of weeks. That's because they're reaching opposition - the point in the celestial scheme of things when a celestial body is directly opposite the sun, as seen from Earth. That's generally the best time to see any planetary object because it's relatively big and close.

"Big and close" is not a term you often hear applied to Ceres, and especially to Pluto. But if you have a chance to see these dwarfs, particularly through a telescope that's big and close, this is the time to do it.

Ceres comes into opposition tonight in the constellation Sagittarius, and should be visible as a magnitude-7.2 object. That's not bright enough to see with the naked eye, but a good pair of binoculars or a small telescope should bring it into view. Gaherty recommends using a star atlas or planetarium software to check the position, and looking for it a couple of nights later to make sure the object you were seeing has moved with respect to the background stars.

Pluto is more of a challenge: It comes into opposition on June 25, when the moon is nearly full and close to Pluto's position in the sky. Moreover, Pluto is much dimmer - magnitude 14, which is why it was such a challenge for Clyde Tombaugh to discover the darn thing 80 years ago. Tombaugh found it while poring over photographic plates made with a 13-inch telescope, and you'll need a telescope with a mirror about the same size to see it this month. You'll also want to dredge up a detailed finder chart. The easiest thing to do might be to buddy up with your local astronomy club and cajole somebody with a big scope into giving you a look.

Or you could wait until 2015, when the Dawn probe (heading for Ceres) and the New Horizons probe (heading for Pluto) are due to make their closest approaches. The New Horizons probe was awakened from its slumber a few weeks ago for a thorough checkup. Just this week, the spacecraft passed the halfway point between Earth's location at the time of its 2006 launch and Pluto's projected location for the 2015 flyby. Today NASA Science News published a nice update on New Horizons' progress and what lies ahead.

One quibble I have with Gaherty's report is that he defines a dwarf planet as "a solar system object which is too small to qualify as a planet." Actually, dwarf planets are indeed big enough to be planets, because they're massive enough to crush themselves into balls through self-gravity. That's the definition the International Astronomical Union came up with four years ago, during a contentious meeting in Prague. According to the IAU, the distinction is that a dwarf planet has not "cleared the neighborhood of its orbit." Does that mean dwarfs should be ruled out as "real" planets? The IAU says yes, but I've tried to make the case that Pluto and Ceres are just different types of planets. You can read all about it in my book, "The Case for Pluto."

Another dwarf planet, Haumea, came in for a shout-out this week when astronomers reported their estimates for the size and brightness of an icy object that was apparently struck off Haumea long ago. The cleverest part of the observation was that they made their estimates based on how the object, known as 2002 TX300, blocked out the starlight shining from behind it. I ran that research past Caltech astronomer Mike Brown, the leader of the team that discovered Haumea in 2005, and here's part of what he said in his e-mailed reply:

"It's a really pretty paper. The light curve from Maui is just jaw-dropping. People have been trying this sort of thing for a long, long time and everyone has wondered when the first one would pay off. ...

"It's pretty clear that objects this small have no viable resurfacing mechanism, so the only solution is that fresh water ice is able to retain its fresh appearance in spite of bombardment. We've basically had to accept that for a while now, with all of the small water-icy Haumea family members around. TX300 is on a nicely stable orbit, so it should be around pretty much until the end of the solar system. No comet fun, like Haumea itself might get to do.

"All in all, a fun paper. I love this sort of astronomy where it takes the work of many people and many small telescopes rather than the pounding by one big telescope. A nicely satisfying result, and the fact that the clincher came from the parking lot half way up the mountain on Mauna Kea is just fabulous!"

Check out "Mike Brown's Planets" to get his perspective on 2002 TX300 as well as Haumea and the other dwarf planets, plus the reasons why he thinks they should not be considered real planets. Later this year you'll also be able to read his book, "How I Killed Pluto and Why It Had It Coming" ... but please, read mine first.

Update for 3:15 p.m. ET June 20: More perspectives on dwarf planets may be on the way ... longtime Plutophile Laurel Kornfeld says she's working on a book titled "The Little Planet That Would Not Die: Pluto's Story." She references her project in the comments below and includes a link to her Live Journal blog. Mark Sykes, director of the Arizona-based Planetary Science Institute and a member of the Dawn science team, has long talked of writing a book about Ceres, "The Littlest Planet." You can hear Sykes discuss the subject in this "Nova" podcast.


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