It's one of the most famous sci-fi scenes in cinematic history: Luke Skywalker standing in the desolate surroundings of his home watching the twin sunset on his homeworld of Tatooine in "Star Wars: A New Hope." Tatooine, a habitable world (or it was until the Rakata race plundered the planet of its natural resources), exists in the fictional binary star system of Tatoo.
But how realistic is it to have a stable planet orbiting one (or both) of the stars in a binary system? According to some fascinating research led by astronomers at Tennessee State University, such a double-sunset scene may not be too farfetched.
By developing a technique called precision astrometry, the researchers have discovered a Jupiter-mass exoplanet orbiting the primary star of the binary system of HR 7162 (or HD 176051), 49 light-years from Earth. They've called it "Inrakluk."
Astrometry is the study of the precise measurements of the positions and movement of stars. This is the first time the technique has been used to decipher the presence of an exoplanet in the complex dynamics of a binary system. The periodic variation in location of HR 7162 led to the Jupiterlike exoplanet's discovery.
But the very existence of Inrakluk has called into question our understanding about how planetary systems form.
In a one-star system like our solar system, the leading theory as to how the planets formed is through a process called accretion. The young solar system would have looked vastly different than it does today; a juvenile sun surrounded by a disk of gas and dust. Over the course of millions of years, this accretion disk would have become more "clumpy." As it cooled, the dust would stick together, slowly forming the rocky seeds of planetary cores.
These protoplanets would gradually exert a gravitational pull on the local volume of space, pulling in more and more material. Eventually, the planets came into being, vacuuming up the rest of the planet-building debris surrounding the sun.
This is all well and good for a single star, but what if you throw a second star into the mix, like in the HR 7162 binary? The accretion model for planetary formation simply doesn't stick.
When there's a second star orbiting the primary (more massive) star in the binary, according to planetary evolution models, the combined gravitational tugging of both stars hinder the slow accretion of the planets. The planet-building materials are ejected from the system before planets can form. This is one reason put forward that may explain why so few exoplanets have been discovered in binary systems.
However, this new discovery proves that mature Jupiter-type worlds can exist in binaries. The TSU-headed group have challenged the planetary accretion theory as the sole planet-forming mechanism in favor of a faster "gravitational collapse" model.
Models predict that binary stars can create a very turbulent environment for planetary formation. Some regions in these systems could become "overdense." Large collections of dust are corralled together where the mutual gravity of the cloud causes it to rapidly collapse. In this case, the formation of Jupiter-mass worlds may only take a few thousand years to form, well before the planet-building materials are ejected from the system.
But where does that leave Tatooine? Well, for starters, Inrakluk is a gas giant, so there's no surface for Luke to stand on to admire a double sunset. Also, the two stars depicted in Luke's double sunset appear to be close together, so the system of Tatoo is most likely a compact binary where the stars orbit very closely. According this research, the exoplanet orbits the primary star, whereas the binary partner orbits the pair. But that doesn't mean a double sunset isn't possible if the alignment is right.
According to Matthew Muterspaugh, a TSU astronomer and the study's lead scientist, his team's development of astrometry could be refined to hunt for Earthlike exoplanets:
"The study of planets and their origins is important for many reasons to humanity. Understanding that there are other planets like our own, helps us better understand how our own planet came to be and how we, as human beings, came to be and where we are going. The techniques we’re developing could help us better locate Earthlike planets in our local neighborhood in the galaxy."