Astronomers using an array of 66 radio telescopes have discovered what may be the first example of a star with two exoplanets circling it that share the same orbit, something that was thought to be theoretically possible but never seen until now.
If you read enough science fiction or bad pseudoscience, sooner or later you'll run across the idea that there's another planet sharing the same orbit as the Earth, but 180 degrees away, so it is always hidden by the Sun.
It's a concept that inspired the 1969 feature film Doppelganger and the appalling Gor paperback series, but it's more fantasy than science. Leaving aside that the Earth changes speed as it orbits the Sun, which would make the alternate Earth visible at times, another planet in such a spot would be unstable because of the combined gravitational pull of the Sun and the Earth forcing it into a new orbit.
However, it is possible for more than one planet to share an orbit if they are in the right position, which would be one of the Lagrange points 60 degrees ahead or behind the primary planet, where the gravitational pull of the Sun and the primary balance one another out, producing a relatively stable area.
The most famous example of this are the Trojan points in the orbit of Jupiter, which are home to 11,552 asteroids. Venus, Earth, Mars, Uranus, and Neptune have similar captured asteroids, though these amount to only a handful.
Two decades ago, it was predicted that other solar systems may have similar configurations and may even have whole planets sharing the same orbit. To confirm this, an international team of scientists formed the Troy project, which has been using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to collect radio emissions from the very young dwarf star PDS 70, located 370 light years away in the constellation of Centaurus.
What they found were two Exo-Jupiter planets called PDS 70b and PDS 70c. This isn't too unusual. However, in the Trojan point next to PDS 70b there was a cloud of debris with about twice the mass of our Moon.
Exactly what this cloud is isn't certain, but it could be a forming planet or conceal one already formed. As to whether it is really sharing the same orbit as PDS 70b will remain unconfirmed until 2026 when the two bodies have moved far enough in their orbits to allow their trajectories to be plotted.
The research was published in Astronomy & Astrophysics, and the video below discusses the Exo-Trojan system.
Source: ESO
