Dimmest Star-Like Body Found Near Earth

Aug. 8, 2006 — Don't look now because you can't see it anyway. Astronomers have pinpointed the faintest object yet detected beyond our solar system: a brown dwarf 100 million times dimmer than the Sun.

At just 16.2 light years from Earth, the dwarf — named DEN 0255-4700 — is not just incredibly dim, but also nearby.

Its discovery is part of an effort to understand the Milky Way at large by first getting a better idea of what's in our own neighborhood, a sphere about 160 light years wide.

"It's a transitional object between a star and a fully-fledged planet," said astronomer Rene Mendez of the Universidad de Chile in Santiago.

DEN 0255-4700 lacks the mass, and therefore the gravity, to generate the thermonuclear reactions that make a true star, he explained.

In fact, its surface temperature is only 2,600 degrees F. While that beats New York's recent surface temperatures, it's downright frigid compared to the 4,000 degrees of the smallest stars.

Mendez and Edgardo Costa led the team that made the discovery, which will be published in the September issue of Astronomical Journal.

"The fact that it’s the faintest and the nearest go together," said Mendez. If it wasn't so near, it would not have been found. And without detecting bodies like it, scientists would have no way to know how common such objects are throughout the Milky Way.

That's exactly what makes DEN 0255-4700 so important.

"Basically we are trying to get a better gauge of the solar neighborhood,” said Mendez. "It's not very well known. There are a lot of stars and stellar systems missing."

Two trails of evidence hinting at those missing stars, says Mendez.

First, the map of known stars in our neighborhood shows there are far more in the northern skies than to the south.

That's due to a longer history of telescopes and observations in the Northern Hemisphere, Mendez explained. So it's likely that many stars in southern skies are missing from the local stellar census.

"In the stellar neighborhood it is more likely that the (stellar) population is homogenous," said Mendez. "There is a lot of space still to make discoveries in southern regions."

The second string of evidence for those missing stars is where DEN 0255-4700 comes in. After counting up our immediate neighbors — those dim stars and brown dwarfs within about 17 light years — astronomers can extrapolate outward to see if the numbers drop off in proportion to distance.

If they do, it's likely that the stars are out there, but not yet charted.

"The further away you go, the less stuff you see that is faint," said astronomer Todd Henry of Georgia State University in Atlanta.

The existence of DEN 0255-4700, Henry said, increases the likelihood that there are many more objects like it littering interstellar space like dim confetti.

The existence of those objects, in turn, would account for more of the mass in the main body of the Milky Way, he said.

The more mass that can be accounted for, the less astronomers need to call on mysterious stuff, such as the gravity of unseen "dark matter," to explain the Milky Way's shape, size and movement.

"I still don't think we have the answers," said Henry, but DEN 0255-4700 appears to be one part of them.