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“Failed star” orbits Sun look-alike

PZ Tel A and PZ Tel B

The Sun-like star, PZ Tel A and its brown dwarf companion, PZ Tel B. The majority of light from PZ Tel A has been blocked using specialised image analysis techniques. For distance comparison, the size of Neptune's orbit is shown.

  • Brown dwarf, a “failed star”, spotted
  • 36 times the mass of Jupiter
  • Orbits a younger version of our Sun

Astronomers have made a direct image of brown dwarf in a close orbit around a young, Sun-like star. Brown dwarfs are a class of astronomical bodies that are bigger than planets but smaller than genuine stars. They’re often called “failed stars”.

The team was led by Beth Biller and Michael Liu from the University of Hawaii (UA). They used the huge 8-metre Gemini South telescope in Chile, which is operated by a consortium of countries, including Australia.

Dubbed PZ Tel B, the brown dwarf was spotted at a distance of only 18 astronomical units (AU) from its parent star, known as PZ Tel A.

An astronomical unit is a standard measurement used by astronomers, being the average distance between the Sun and the Earth. At 18 AU, PZ Tel B is at the equivalent of the orbit of Uranus in our Solar System.

The brown dwarf is not visible in an image made back in 2003, suggesting it was at that time closer to and lost in the glare of its parent star.

“Because PZ Tel A is a rare star being both close and very young, it had been imaged several times in the past” said Laird Close, a professor at UA’s Steward Observatory. “So we were quite surprised to see a new companion around what was thought to be a single star.”

The new observations confirm that the brown dwarf is currently moving outward from the main star. They also show that it is 36 times the mass of Jupiter, the largest planet in our Solar System.

“PZ Tel B travels on a particularly eccentric orbit—in the last 10 years, we have literally watched it careen through its inner solar system,” said Beth Biller, lead author of the scientific paper. “This can best be explained by a highly eccentric, or oval-shaped, orbit.”

Brown dwarf size compared to Jupiter, the Sun and the Earth

The size of a brown dwarf compared to Jupiter, the Sun and the Earth (to scale). Brown dwarfs are more massive than planets and less massive than stars, but have similar diameters to planets such as Jupiter.

A young version of our Sun

The host star, PZ Tel A, is similar to our Sun, but at 12 million years of age is about 400 times younger. Astronomers are keen to study it and other such stars to learn more about the formation and evolution of Sun-like stars.

PZ Tel A is expected to retain a surrounding cloud of gas and dust from which planets might form. The gravitational pull of the brown dwarf could upset the formation of any such planets.

The find was made using the Near-Infrared Coronagraphic Imager (NICI) instrument, which blocks out much of the glare of a star and enables nearby regions to be seen.

The brown dwarf is so close to its parent star that it required all the power of NICI, plus adaptive optics—which help to remove the blurring effect of the Earth’s atmosphere—plus special image enhancing techniques, to pick it out of the glare.

NICI is so powerful that it can detect objects 1 million times fainter than their host stars at very close distances.

An international team is using NICI to conduct a 300-star survey, and it will be fascinating to see what they find.

“We are just beginning to glean the many configurations of solar systems around stars like the Sun,” said NICI Campaign leader Michael Liu. “The unique capabilities of NICI provide us with a powerful tool for studying their constituents using direct imaging.”

Also involved in the PZ Tel B research were graduate students Eric Nielsen, Jared Males and Andy Skemer.

Story by Jonathan Nally, Editor,

Images by Jon Lomberg / Gemini Observatory / Beth Biller / Gemini NICI Planet-Finding Campaign.

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