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Title: Weird Planet Is Orbiting Backwards Between Two Stars

Link: https://science.slashdot.org/story/25/05/23/0...

Astronomers have confirmed the existence of a bizarre planet in the Nu
Octantis binary star system that orbits in reverse between two stars -- one
of which is now a white dwarf. This retrograde orbit, once thought
impossible, defies traditional planetary formation models and may have
resulted from dramatic shifts in the system's history. New Scientist reports:
The key observation was that the Nu Octantis planet is retrograde -- the
planet and one star both orbit the second star, but they do so in opposite
directions, with the planet having the tighter orbit around the second star.
[Man Hoi Lee at the University of Hong Kong] says this is unusual but makes
the system's configuration stable -- even though it means that the planet
repeatedly moves through the narrow space between the two stars. His team was
able to determine this with lots of certainty thanks to improved measuring
devices, such as the HARPS spectrograph at the European Southern
Observatory's 3.6-metre telescope in Chile. The fact that the planet's signal
persisted through years of observation helped too. "We are pretty sure [the
planet] is real, because if it was something like stellar activity, it
shouldn't be so consistent in years of data," says Lee. But this backwards-
moving planet isn't the only exotic feature of Nu Octantis. The researchers
used the European Southern Observatory's Very Large Telescope, also in Chile,
to determine that one of its stars is a white dwarf, which means that it has
reached the end of its life cycle, becoming denser and smaller. Lee says this
complicates the Nu Octantis threesome's history because mathematical models
of its past show that the planet's current orbit was impossible when this
star was younger, bigger and brighter. So, the planet either used to orbit
both stars at once, but then radically shifted trajectory when one of the two
stars became a white dwarf, or it was formed from the mass that the star
ejected as it transformed into a white dwarf. Future observations, and a lot
more mathematical modelling, may be able to pinpoint which of these scenarios
is more likely to have occurred, but both are rather novel, says Lee. The
research has been published in the journal Nature.

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