A Mysterious Planet Revolves Counter to Its Stars

Following extensive observation, scientists are on the verge of unraveling how pairs of stars engage in stable orbital dynamics surrounding elusive planets.

In 2004, David Lamb from the University of Canterbury, New Zealand, identified a puzzling repeating signal while monitoring the motion of a star pair in the Nu Octantis system. This initiated an ongoing discussion about whether planets twice the size of Jupiter exist in that system. Now, along with Ram Mann Whiley from the University of Hong Kong and his colleagues, they present strong evidence suggesting that Nu Octantis is a trio rather than a binary system.

A significant discovery was that the Nu Octantis planet is moving in reverse. The planet and one star orbit the second star in opposite directions, with the planet maintaining a close orbit around the latter. Lee observes that this is an unusual occurrence, but the system is stable. His team reached this conclusion thanks to enhanced measurement tools, like the HARPS spectrometer on the 3.6-meter telescope at the European Southern Observatory in Chile. The persistence of the planetary signal across years of observation reinforced their findings. “We’re pretty sure [the planet] is genuine. If it were related to stellar activity, it shouldn’t exhibit such consistency over years of data,” remarks Lee.

Nonetheless, this retrograde planet is not an uncommon feature of Nu Octantis. Researchers utilized a large telescope at the Southern European Observatory to determine that one of the stars is a white dwarf. Lee explains this complicates the history of Nu Octantis, as it suggests that the planet’s current orbit was impossible when it was younger, larger, and brighter.

Thus, the planet initially orbited both stars simultaneously but fundamentally changed its trajectory when one of the stars became a white dwarf, or it formed from a mass expelled when the stars transitioned to white dwarfs. Continued observations and mathematical modeling may clarify which scenario occurred, but both possibilities are novel, notes Lee.

For centuries, astronomers believed that all planets orbit the central star in the same direction, with regular intervals governing the orbital arrangement. However, Nu Octantis challenges these conventions, according to Manfred Kunz from the University of Texas at Arlington. “Scientists are urging us to broaden our understanding of star and planetary scenarios, in terms of both formation and evolution,” he states.