New planet-hunting tool chases after star system's little brown dwarf

The Sphere instrument on the VLT in Chile peers with all its might into the blackness around a binary star system and returns a surprise for astronomers.

Michael Franco
Freelancer Michael Franco writes about the serious and silly sides of science and technology for CNET and other pixel and paper pubs. He's kept his fingers on the keyboard while owning a B&B in Amish country, managing an eco-resort in the Caribbean, sweating in Singapore, and rehydrating (with beer, of course) in Prague. E-mail Michael.
Michael Franco
3 min read

The Sphere instrument, in the black box on the side of the telescope, is known as a planet hunter. Now it can add "dwarf dispeller" to its resume. ESO/J. Girard

Usually, stories related to the Very Large Telescope array -- a collection of four telescopes built by the European Southern Observatory in Chile -- have to do with astounding things the high-powered lenses have discovered. On Wednesday, the VLT made news for something it failed to spot -- namely, a brown dwarf star that was believed to have been orbiting a binary star system known as V471 Tauri, in the Taurus constellation.

One of the two closely orbiting stars that make up V471 Tauri is a white dwarf and the other is a normal star like our sun, according to a report about the star system on NASA's Chandra X-ray Observatory's website. The binary system is located about 163 light-years from Earth and the distance between the two stars is one-thirtieth the distance between Mercury and the sun, which means they make a full revolution around each other every 12 hours.

The sky around V471 Tauri. According to ESO: "The object itself is visible as an unremarkable-looking star of moderate brightness at the centre of the image." ESO/Digitized Sky Survey 2

When either star passes in front of each other, astronomers can read the changing light signals from the system. A team of led by Adam Hardy of Chile's Universidad Valparaiso measured the changes in brightness down to a very accurate 2 seconds by using the Ultracam system on ESO's New Technology Telescope. The Ultracam is a superfast camera attached to the VLT that can take up to 500 pictures per second in three different colors simultaneously.

The researchers discovered that the eclipses between the two stars didn't happen at a regular interval like they anticipated. To explain the anomaly, astronomers have theorized that there must be a brown dwarf orbiting the system and warping the orbit of the binary stars with its gravitational influence. Brown dwarfs, which are sometimes described as "failed stars," form when a cloud of gas and dust begins to congeal around a central core as in star formation. But in the case of the dwarfs, the material at the core never gets packed in tightly enough to start nuclear fusion and lead to a proper star. So these brown stars float through the galaxy -- not quite stars, not quite planets.

Still, a brown dwarf would have enough of a gravitational pull to explain the odd orbit in V471 Tauri. Until recently, however, it was impossible to actually look for the dwarf, as they are very dim celestial bodies.

An artist's rendering that provides a reference point for the size of a brown dwarf. NASA/JPL-Caltech/UCB

The new Sphere instrument on the VLT could see such a thing. "Sphere is a powerful planet finder and its objective is to detect and study new giant exoplanets orbiting nearby stars using a method known as direct imaging," ESO explains. "In other words, Sphere is trying to capture images of the exoplanets directly, as though it were taking their photograph."

So, scientists pointed Sphere at the binary stars and found...absolutely nothing. The brown dwarf that was supposed to be there simply isn't.

"The Sphere images are so accurate that they would have been able to reveal a companion such as a brown dwarf that is 70,000 times fainter than the central star, and only 0.26 arcseconds away from it," says ESO. "The expected brown dwarf companion in this case was predicted to be much brighter."

So if it's not a brown dwarf influencing the orbit of the stars, then what is it?

One of the more popular theories is that it could be magnetic variation in one of the stars, similar to what happens to our very own star. Such an effect is called the Applegate mechanism, according to ESO, and it makes the shape of the star actually change, which could alter the apparent brightness of the double star seen from Earth.

But more important than knowing exactly what's influencing V471 Tauri's strange orbit is the fact that the Sphere device helped astronomers rethink the way they theorize about the objects they observe.

"This is how science works: observations with new technology can either confirm, or as in this case disprove, earlier ideas," Hardy says. "This is an excellent way to start the observational life of this amazing instrument."