Barnard's Star's planetary layout may preserve signs of a lost world

Barnard's Star has four known planets, but their current arrangement may reflect the loss of an additional world. A past disruption could have brought two planets together to form the present-day planet c. Two different processes could explain how the system's planets moved into their current positions, and a comparison with other compact systems finds a similar pattern in many cases.

Star system: Barnard star | Distance from the Sun: 6 light years

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Barnard's Star's planetary layout may preserve signs of a lost world

Star system: Barnard star | Distance from the Sun: 6 light years

The arrangement of the four known sub-Earth-mass planets around Barnard's Star may record the loss of an additional planet. In this possible history, a disruption broke a sequence of orbital resonances and led two planets to merge, forming the present-day planet c. A resonance occurs when orbiting bodies repeatedly line up in the same pattern because their orbital periods are related. The current layout is consistent with this kind of broken resonant chain, but it does not establish that a lost planet or merger definitely existed.

Two possible processes could have shifted the planets into the observed arrangement. In one scenario, tides raised by the star gradually expanded the planets' orbits over a long period. In the other, movement through the early planet-forming disk expanded the system. In that case, one-sided Lindblad torques - gravitational pushes from the disk acting more strongly on one side of a planet - separated the two outermost planets. Both pathways can produce conditions for the merger of the two bodies that became planet c.

The disk-driven pathway also resembles a pattern found in other compact systems. Of 14 identified four-planet systems with broken resonant-chain layouts similar to Barnard's Star's, ten have the third planet interrupting the sequence. This comparison shows that such a break is common in this selected group, though it does not by itself establish how any individual system formed.

Key findings
Barnard's Star has four known sub-Earth-mass planets in an arrangement consistent with a broken resonant chain.
A proposed past merger between two planets could have formed the present-day planet c, possibly after an additional planet was lost.
Long-term tidal expansion driven by the star and early expansion driven by the planet-forming disk are both viable pathways.
In a group of 14 similar compact four-planet systems, ten have the third planet breaking the resonant sequence.

Scientific study

Breaking the Chain: The Dynamical History of the Barnard's Star Planetary System