June 15, 2024

Two planets in the same orbit

About 370 light-years away, in the infant star system PDS-70, astronomers may have just caught an exoplanet pulling together in the same orbit as its massive sibling.

We already know that more than one object can share the same orbit; Jupiter, for example, has a collection of 120,000 asteroids following the same paths around the Sun. Earth has one too. But while it’s theoretically possible, astronomers have never found two complete planets sharing the same orbit around a star before.

“Who could imagine worlds that share the length of the year and the habitable conditions? Our work is the first evidence that this kind of life could exist,” he says Olga Balsabre-Ruza of the Astrobiology Center in a recent statement. She and her colleagues published their findings in the journal Astronomy and Astrophysics.

This Gas Giant will have a Little Friend – and some Moons

The large solid line shows the path of PDS-70b around its star. The solid line circles the smaller PDS-70b itself, and the dotted line circles the newly observed debris cloud.


PDS-70 is a very young star system, only about 5.4 million years old, so young that planets are still condensing from the disk of gas and dust around the small star. It is one of the few exoplanet systems that astronomers have been able to directly image; from Earth, we appear to be looking straight up (or straight down) into the system. At least two planets, both gas giants, have been formed so far: PDS-70b and PDS-70c. There may be others soon; astronomers can see gaps in the dust forming the disks, which may mark the orbits of planets at different stages of formation.

Balsalobre-Ruza and her colleagues recently saw a cloud of debris about twice the mass of our Moon creeping just behind the innermost gas giant, PDS-70b. The debris could be the building blocks of a new dwarf planet, or it could be a cloud around a young planet that has already formed. PDS-70b itself is so young that it is still surrounded by its small disk of gas and rocky debris, which will eventually merge with the moon (it still has a disk of lunar material swirling around it) PDS-70c as well.

The debris cloud is in the right place for what astronomers call a Trojan object (the name comes from the band of asteroids below Jupiter, mentioned above, which was originally named after the Trojan heroes from the Iliad; now it is a catch-all term for any person). objects that share an orbit in the same way). It is in an area known as the Lagrangian, a region in a planet’s orbit where objects – such as asteroids – can be trapped due to the balance between the planet’s gravity and the star’s gravity.

“[Exoplanet Trojans] until now they’ve been like unicorns: They’re allowed to exist in theory, but no one has ever detected them,” says Coauthor Jorge Lillo-Boxalso from the Center for Astrobiology, in a recent statement.

As if the orbital situation in PDS-70 wasn’t complicated enough, PDS-70b and PDS-70c orbit nearly 1:2, meaning that PDS-70b completes about one orbit in the time it takes its closest neighbor. peripherals. , PDS-70c, to complete about two orbits. There is every chance that these two planets will draw closer to being in true resonance each time these two planets pass around their own star. And it’s not clear how that might affect the PDS-70b Trojan trailer.

Balsalobre-Ruza and her colleagues discovered the debris cloud in archived data from the Atacama Large Millimeter/sub-millimeter Array (ALMA), a telescope in the Chilean highlands. They will have to wait a few years to confirm that what they found is actually orbiting the star on the same path as PDS-70b, however; It takes about 120 years for the gas giant to make a complete lap around the star, so it will be a few years before the planet, or the debris cloud, has given astronomers enough time to confirm that they are moving together Seriously.

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