A unicorn of astronomy: first planets with identical orbits discovered

Trojan

Orbits are generally thought of as regions dominated by a single main object, although allowing for the passage of several asteroids that ultimately do not have a major impact. For some years now, the possibility of finding two planets in a single orbit around the star has been predicted, however, this phenomenon would be incredibly rare in the universe and unlikely to be detected. Or so it was thought until the first candidate for a coorbital Trojan was found using ALMA.

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Image taken with ALMA. In the center is the star PDS 70, and around the yellow ellipse is the exoplanet PDS 70b (yellow circle) and the debris cloud around L5 (dashed yellow circle). In addition, the large outer band corresponds to a ring of material where new planets are forming. Credits: ALMA (ESO/NAOJ/NRAO)/Balsalobre-Ruza et al.

Gravitational equilibrium

Planetary systems are a delicate gravitational balance between the different bodies that compose it. During millions of years of evolution and formation, the interactions between planets forge the orbits and determine the location and trajectory of minor objects. By the very chaotic nature of the three- and ene-body problem, there are few stable configurations in time. The most common of these is the one we know from planets in orbits with different distances.

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Diagram showing the relative location of Lagrange points with respect to a star and a planet.

A low-mass object, under the mutual gravitational interaction of a planet and the star, can be trapped in what are known as Lagrangian zones. These are specific points in the orbit where there is an equilibrium between the two attractive forces. A clear example of this is Jupiter and the Trojans, a large swarm of asteroids located around Lagrange points 4 and 5.

A special Trojan

PDS 70 or V1032 Centauri is a young star in the constellation Centaurus approximately 370 light-years from Earth. It has a protoplanetary disk, home to two exoplanets in formation that allowed, thanks to ALMA's high resolution, the first direct image of a protoplanet.

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Color photograph of the region near PDS 70 (located in the center of the image). Credits: ESO/Digitized Sky Survey 2: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin

Thanks to the TESS observatory, two planets of Jupiter-like size and mass were known to exist around the star PDS 70. A team of researchers found a faint signal in the ALMA archive data around PDS 70b, whose origin corresponded to point L5. Giving clues to the presence of a debris cloud with a mass equivalent to twice that of the Moon.

It is believed that this cloud may be the result of the existence or formation of a Trojan of considerable mass in the system. If so, there are two planets around the same star with the same rotation period and habitability conditions. This, despite having been predicted, is still an incredibly strange and striking finding.

Patience in science

It is necessary to clarify that the object is currently cataloged as a coorbital Trojan candidate, the research team expects to make new observations in February 2026, when both bodies should move behind the star, to better determine its nature.. As well as laying the groundwork and providing motivation to continue the search for more Trojan systems in other star systems.

Francisco Andrés Forero Daza