"Dark Sun" Is One of Our Nearest Neighbors

By Ken Croswell

Published on NewScientist.com (April 9, 2010)

A dim object less than 10 light-years from Earth appears to be the closest brown dwarf yet found. The "star" is so cold that any residents on an orbiting planet would see a dark sun in their starry "daytime" sky.

The discovery suggests that brown dwarfs are common and that the objects could exist even closer to Earth.

Brown dwarfs have so little mass that they never get hot enough to sustain the nuclear fusion reactions that power stars like the Sun. Still, they do shine, because they glow from the heat of their formation, then cool and fade.

Philip Lucas of the University of Hertfordshire in Hatfield, UK, and his colleagues discovered the brown dwarf, named UGPS 0722-05, from the infrared radiation it gives off. It is only about 9.6 light-years from Earth, a bit more than twice as far as Proxima Centauri, our nearest star after the Sun.

At that distance, it is the seventh closest star system to the Sun. Not since 1947 have astronomers uncovered a new star so close to Earth.

Rank	Star	Distance	Spectral	History
		(ly)	Type

0

Sun

0

G2

1

Alpha Centauri A

4.37

G2

Parallax measured by Thomas Henderson, 1838

Alpha Centauri B

4.37

K1

Proxima Centauri

4.24

M5.5

Discovered by Robert Innes, 1915

2

Barnard's Star

5.98

M4

Discovered by Edward Emerson Barnard, 1916

3

Wolf 359

7.8

M6

Discovered by Max Wolf, 1918

4

Lalande 21185

8.3

M2

Discovered by Friedrich Wilhelm Argelander, 1857

5

Sirius A

8.6

A1

First accurate parallax measured by David Gill and William Elkin, 1881-1883

	Sirius B	8.6	WD	Reported astrometrically by Friedrich Wilhelm Bessel, 1844
				Discovered visually by Alvan Graham Clark, 1862

6	Luyten 726-8 A	8.7	M5.5	Discovered by Willem Luyten, October 29, 1947
	Luyten 726-8 B	8.7	M6

7?

UGPS 0722-05

9.6?

T10?

"Great stuff!" says Todd Henry, a nearby-star researcher at Georgia State University in Atlanta, who was not part of the team. "This discovery is as cool as its temperature."

Lucas and his colleagues caution that their estimated distance is preliminary. It is based on parallax, which offers a reliable method of deducing a star's distance from Earth: if an observer on Earth measures the star's position in the sky and then looks at it again months later, the star will appear to have moved slightly because it is being viewed from a different point in our planet's orbit around the Sun. Knowing the dimensions of Earth's orbit, astronomers can calculate the star's distance from the amount of its apparent movement.

But at the moment, Lucas and his colleagues don't have good enough parallax measurements to be sure of the brown dwarf's precise distance and could be a light-year or so out. In just a few weeks, however, new parallax observations should pin the distance down.

If the current distance estimate is right, the brown dwarf is closer than any other known. The previous record-holder is a pair of brown dwarfs around the star Epsilon Indi, 11.8 light-years from Earth.

The new brown dwarf breaks two other records as well. It's the coldest brown dwarf ever seen, with a temperature of just 400 to 500 Kelvin (260 to 440 degrees Fahrenheit). And it's the dimmest: it emits only 0.000026 percent as much energy as our Sun, and this energy emerges at infrared rather than visible wavelengths. It would take 3.8 million of these brown dwarfs to equal the Sun's power. It is about the size of Jupiter, but its mass is 5 to 30 times greater.

The object's feeble nature explains why it has only now been spotted, despite its proximity. It was found after surveying only a few per cent of the sky, which implies that many more brown dwarfs are lurking nearby undetected.

The brown dwarf is in Monoceros, a constellation that neighbors Canis Major, the home of Sirius. From Earth, Sirius is the brightest star at night. From any planet orbiting UGPS 0722-05, Sirius is the brightest star, day or night, and looks nearly as bright as Venus does in our sky.

Lucas and his colleagues have submitted their paper to Nature.

Update (June 24, 2010): After reporting in April that a "dark sun" named UGPS 0722-05 was the closest brown dwarf to planet Earth ever found, researchers led by Philip Lucas of the University of Hertfordshire in Hatfield, UK, have revised their claim. Based on new parallax measurements, they now say the object is actually 13.3 light years away, not 9.6 as previously reported--which means a pair of brown dwarfs orbiting Epsilon Indi is closer. The astronomers also raised their estimates of the brown dwarf's temperature--to between 480 and 560 Kelvin (405 to 550 degrees Fahrenheit), from 400 to 500 Kelvin (260 to 440 degrees Fahrenheit)--and luminosity, upping their original estimate that it emits 0.000026 percent as much energy as our Sun to 0.000093 percent.

Lucas and his colleagues have withdrawn their paper from Nature and submitted a revised version to Monthly Notices of the Royal Astronomical Society Letters.

Ken Croswell is the author of Planet Quest, whose seventh chapter opens with these words:

Not far from our solar system may lurk a cold, dark "star," completely invisible to human eye. This ghostly object drifts among the Galaxy's luminous stars, dragging with it a small retinue of planets that enjoy no light or warmth from the star they circle. When the star was young, it did glow dim red, but it had too little mass to ignite its main supply of nuclear fuel. Instead, the star steadily cooled and faded, like an ember plucked out of a fire, and the planets watched helplessly as it turned black and their temperatures plunged to near absolute zero. Billions of years later, these planets still faithfully revolve around their failed star, its gravity anchoring them to its lifeless embrace; and any light that does reach their frigid surfaces comes from beyond--from stars like Sirius, Alpha Centauri, and our own Sun.

KenCroswell.com

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