The Solar System is a very organised place. Each planet is in its own orbit, at its own orbital distance from the Sun, minding its own business.

Most planetary systems we find in the wider galaxy also seem to follow this trend. But according to theory, two planets can share the same orbit, and now, for the first time, astronomers think we may have evidence of this in a baby planetary system about 370 light years from Earth.

These orbit-sharing planets are known as Trojan planets, or exoplanets for exoplanets, after populations of asteroids that share Jupiter's orbit and hover at gravitationally stable points along its orbit.

"Twenty years ago, it was theoretically predicted that pairs of planets with similar masses could share the same orbit around their star; these planets were called Trojan or co-orbital planets. This is the first time we have found evidence in favour of this idea," says astrophysicist Olga Balsalobre-Ruza from the Centre for Astrobiology in Spain.

The system where the discovery was made is already famous: PDS-70. This is the first time we have seen direct images of not one, but two exoplanets in the process of forming baby gas giants called PDS-70b and PDS-70c. Here, too, astronomers found evidence of a moon-forming disc around one of the proto-exoplanets, PDS-70c.

Now, a closer look reveals evidence of something sharing the orbit of PDS-70b - a fainter speck than the exoplanet. Calculations suggest that this thing has about twice the mass of Earth's Moon. And while this blob is not yet a planet, researchers are excited about its location.

It sits at a point known as a Lagrange or Lagrangian point. There are five of these points in any two-body system: three along the line joining the two bodies (L1, L2 and L3) and two (L4 and L5) along the orbit of the smaller body. These are pockets of space where the gravitational interaction between two bodies is balanced by the centripetal force required for a smaller body to move with them.

Jupiter collects asteroids in L4 and L5, collectively known as the Trojan asteroids. There are also several Trojan asteroids on Earth. These are also good places to park space observatories; JWST's home is in one of the Earth-Sun Lagrangians. So it is conceivable that something a little bigger could park in one of these handy spots, but we found little evidence.

It seems appropriate that we should find it in the PDS-70 system, lurking in the L5 Lagrangian.

"Who can imagine two worlds that share the duration of the year and habitable conditions? Our study is the first evidence that such a world could exist," says Balsalobre-Ruza. "We can imagine that a planet could share its orbit with thousands of asteroids, as in the case of Jupiter, but it is inconceivable to me that planets could share the same orbit."

Right now, the orbital partner is probably a thick cloud of dust, the building blocks of a new planet rather than a whole planet. This could help us understand not only the possibility of the formation of exoplanets, but also the formation of planetary systems.

For example, Jupiter is thought to have collected its trojans over time and migrated from a point further away from the Sun. Studying the Jupiter-like PDS-70b could shed light on this theory.

Still, we will have to wait a while for answers. The team will need to look at the system again in 2026 to see if the blob moves with PDS-70b as a co-orbital companion in the L5 Lagrangian.

"This would be a breakthrough in the exoplanet field," says Balsalobre-Ruza.

 

Source: https://www.sciencealert.com/