Free floating planets (which is to say, planet-sized objects floating free in space, unbound to any star) were first observed more than two decades ago, and this population is now known to contain terrestrial mass objects as well as large Jupiter-type planets.
In a paper published in The Astrophysical Journal Letters on 18 December 2023, Amir Siraj of the Department of Astrophysical Sciences at Princeton University, discusses the possibility that one or more terrestrial sized planets might have been captured by the Sun's gravity early in the history of the Solar System, and be as yet undiscovered components of the Outer Solar System.
Siraj notes that this is a different topic to the search for Planet Nine, a hypothetical body with a mass six times that of Earth and a semi major axis (average distance from the Sun) of about 400 AU (i.e. 400 times as far from the Sun as the Earth), which has been proposed due to observed clustering extreme trans-Neptunian objects in the Outer Solar System.
Siraj instead debates the possibility of sub-Earth-mass planets in the Outer Solar System, motivated by the fact that such bodies have been observed free floating in space, and could potentially be captured by the Sun's gravity.
In theory, any stellar system is most likely to capture drifting planets when it is still very young, and within its birth cluster, that is to say a cluster of stars forming within a single molecular cloud, which acts as a stellar nursery. This is the stage at which young stellar systems are most likely to eject planets, and the time when they are close to the largest number of other systems, making it most likely that such planets will be captured.
Stellar nurseries are variable in nature, with planets more likely to be captured in clusters where the molecular cloud is expanding rapidly. To give a conservative estimate of the probability of planet-capture, Siraj assumed a gently collapsing cluster, which is thought to be the environment in which planetary capture is least likely.
Surprisingly, despite applying the most conservative conditions, Suraj's simulation predicts that there wit be approximately 1.2 captured planets with a mass at least equivalent to that of Mars in the Outer Solar System, and 2.4 planets with a mass equivalent to Mercury or larger. If less conservative assumptions are made, this increases to roughly 2.7 planets with a mass equal to that of Mars or larger, and about 5.2 Mercury sized or larger planets. The average distance from the Sun of these planets would be 1400 AU, with half of all such bodies orbiting at between 600 AU and 3500 AU.
Detecting such planets would be another problem, as they would be very faint objects, and we do not actually know where to look for them. The Legacy Survey of Space and Time project at the Vera C. Rubin Observatory is due to Survey the entire Southern Hemisphere sky every three nights in six optical bands ranging from 320 to 1050 nm for a ten year period.
This survey should be capable of detecting such planets in the Outer Solar System, if they are present and visible from the Southern Hemisphere (bodies in the Outer Solar System will orbit extremely slowly, taking hundreds or even thousands of years to complete a single orbit of the Sun, and are also likely to have more eccentric orbits than the planets of the Inner Solar System, making it possible that bodies could spend an entire ten year period in the northern sky). Suraj estimates that this survey could detect between about 1.0 and 1.4 Mercury sized planets, and 0.7-0.9 Mars sized planets, although only planets in the innermost part of the Outer Solar System, between about 400 AU and about 700 AU from the Sun.
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