Thursday, May 31, 2007

Brown Dwarf Is Cool


By now everyone has heard the controversy whether Pluto is a planet. At the other end of the planetary mass scale another controversy is brewing, the boundary between large planets and brown dwarves. The latter are objects between 10-80 Jupiter masses, not big enough for the nuclear reactions of a star but still giving off heat through some mysterious process. Just last month a brown dwarf pulsar was discovered giving off beams of radiation.

Until recently the very existence of brown dwarfs was just a theory. Thanks to infra-red telescopes, we now have discovered many. Yesterday astronomers from the UK Infrared Telescope and Gemini South observatory announced discovery of ULAS J0034-00, the coolest solitary brown dwarf yet found. Its surface temperature is only about 650 degrees, and its mass is between 15-30 times that of Jupiter. It is located approximately 50 light-years away in the constellation Cetus, alone in Space without a parent star. There may be many other similiar objects out there.

Also at the American Astronomical Society meeting in Honolulu, another group of astronomers announced discovery of exoplanet XO-3b. Until a few years ago the existence of extra-solar planets was doubted, like a changing speed of light. With a faith born in science fiction, astronomers today have found hundreds. Having a mass of 13 Jupiters, X0-3b occupies a boundary between big planets and brown dwarfs. It orbits its star in less than 4 days, the largest object yet found in so close an orbit.

It was thought that an object so close to a star would have a circular orbit, otherwise tidal forces would tear it to bits. Not knowing this, X0-3b has taken a highly elliptical orbit. Something else holds this object and other "hot Jupiters" together. Presence of a central Black Hole would explain why Jupiter gives off twice as much radiation as it receives from the Sun, and why brown dwarfs give off heat.

We must also thank the wondrous Kea for pointing out discovery of TWO planets around metal-poor star HD 155358. Once it was thought that heavy metals are necessary for the cores of planets to accrete. HD 155358 formed so long ago (10 billion years!) that it has almost no metals. As we have now discovered many extrasolar planets, new theories are needed.

Yet another group of astronomers today announced discovery of TrES-3. This transitting planet orbits a star in the constellation Hercules with a period of only 31 hours! It surface temperature is about 1500 degrees Kelvin. Old theories can not explain why this planet doesn't boil away. If planets and dwarfs formed around singularities, that would explain how they stay together.

Just as the line between Pluto and planets has become blurred, the boundary between big planets and small stars is being eaten away. It is better to think of a continuous mass scale from minor planets to gas giants, small stars and beyond. The same processes govern the formation of all these objects. If you want to find a nearby Black Hole, look beneath your feet.

Check out the new Carnival of Space!

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3 Comments:

Anonymous Sam said...

Aloha Louise,

ULAS J0034-00 goes some way to pinning down where the threshold lies between what is considered a brown dwarf and what is considered a high-mass planet.

I have however noticed that the universe tends not to respect whatever the taxonomy laid down by the likes of the International Astronomical Union currently is.

Great stuff.

Greetings from UKIRT, thanks for the plug.

3:20 PM  
Blogger Kea said...

Thanks for the plug. It is hard to keep up with new observations!

4:13 PM  
Blogger L. Riofrio said...

HI Sam, the Uniuverse doesn't pay much attention to what the astronomers think.

Always great to hear from yu, Kea. There are so many exciting things happening in astronomy that one blog can't cover them all. Your post on ellis was nice too.

6:49 AM  

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