Bizarre planet orbits backward around a dying star

Most stars are not alone. They live in binary or multiple star systems. These close companions often disturb the formation and survival of planets, but an unusual planet has defied that logic.

An international team led by Professor Man Hoi Lee and MPhil student Ho Wan Cheng at the University of Hong Kong has confirmed a planet in the binary star system Nu Octantis. What makes it unique is its retrograde orbit – moving opposite to the binary’s motion.

Planet found in Nu Octantis system

The system Nu Octantis consists of two stars. The primary, Nu Oct A, is a subgiant about 1.6 times the Sun’s mass. The secondary, Nu Oct B, has about half the Sun’s mass. The stars complete an orbit every 1,050 days.

In 2004, Dr. David Ramm found a strange signal while pursuing his Ph.D. at the University of Canterbury. The signal pointed to a possible planet nearly twice Jupiter’s mass. It orbited nu Oct A every 400 days.

But its orbit could only be stable if it was retrograde. That idea went against accepted theories and lacked observation support.

Planet moves in the wrong direction

To settle this puzzle, the team used new high-precision data from the HARPS spectrograph at the European Southern Observatory (ESO). Their results confirmed the planet’s presence.

“We performed an analysis of the new and archival radial velocity data spanning 18 years and found stable fits that require the planetary orbit to be retrograde and nearly in the same plane as the binary orbit,” said Cheng. This marks the first confirmed case of such a planet.

Nu Octantis has a white dwarf star

Understanding Nu Oct B was key to solving the mystery. Its mass allowed for two possibilities – it could either be a small main-sequence star or a white dwarf.

Most stars, like our Sun, spend their lives fusing hydrogen into helium. After exhausting their fuel, they collapse into remnants. If their mass is below a certain limit, they become white dwarfs. These are Earth-sized but have mass similar to the Sun.

To investigate Oct B’s nature, the team used SPHERE at ESO’s Very Large Telescope. It didn’t detect the star, which meant Nu Oct B is extremely faint – a white dwarf.

Nu Octantis over billions of years

The lack of light from Nu Oct B confirmed that the system had changed dramatically over billions of years. The team then explored how this binary started out.

“We found that the system is about 2.9 billion years old and that Nu Oct B was initially about 2.4 times the mass of the Sun and evolved to a white dwarf about 2 billion years ago,” said Cheng.

“Our analysis showed that the planet could not have formed around Nu Oct A at the same time as the stars.” This meant the planet had to form later, after Nu Oct B became a white dwarf.

A second-generation planet?

The study opened new ideas about planetary formation. According to Professor Man Hoi Lee, the planet may have formed from material ejected by Nu Oct B as it evolved.

Or it could be captured from a prograde orbit around the binary into a retrograde orbit around Nu Oct A, noted Professor Lee.

“We might be witnessing the first compelling case of a second-generation planet; either captured, or formed from material expelled by Nu Oct B,” said Dr. Trifon Trifonov of Zentrum für Astronomie der Universität Heidelberg and Sofia University St. Kliment Ohridski. “It lost more than 75 percent of its primordial mass to become a white dwarf.”

New techniques, new discoveries

“The key to this exciting discovery was the use of several complementary methods to characterize the system in its entirety,” said Dr. Sabine Reffert of Zentrum für Astronomie der Universität Heidelberg.

The team combined long-term radial velocity data with modern imaging methods. They used two ESO-operated instruments: the HARPS spectrograph and SPHERE.

This study shows that planets can exist in unlikely places. Systems with evolved stars may still host planets. Astronomers will likely find more such oddities as their tools and methods improve.

The study is published in the journal Nature.

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