The young sun may
have captured several Mars- or Mercury-size exoplanets that now orbit in the
outer reaches of the solar system, but identifying them will be extremely
challenging.
Rocky planets captured by the sun may reside in the
outer reaches of the solar system
(Credit: da-kuk via Getty Images)
Most astronomers
agree there are only four terrestrial, or rocky, planets in our solar system, all of
which lie close to the sun. But new research suggests that up to five more may
be lurking in the outer reaches of the solar system.
Free-floating
planets (FFPs) are planet-size objects that don't orbit a star. Also called
rogue planets, FFPs either form from clumps of gas unconnected to any
star or arise around stars but get flung out of their home orbits. The James Webb Space
Telescope (JWST) has
identified hundreds of rogue planets in the Milky Way and beyond,
including waltzing
pairs of Jupiter-size planets in the belly of the Orion
constellation.
While some rogue
planets are ejected by stars, stars could also reel in these lonely wanderers
with their gravity, making the planets permanent orbiting members of a solar
system. In the new study, published on Dec. 18, 2023 in the Astrophysical Journal Letters, a researcher proposes that
our own sun, in its infancy, may have lured rocky FFPs too.
To determine the
likelihood of this, study author Amir
Siraj, a doctoral candidate in Astrophysics at Princeton University and
director of the university's Interstellar Object Studies program, used several
previously developed models, or sets of equations, built on observations of
FFPs. Assuming our infant star had a 1-in-50 chance of trapping a rogue planet
— which, he noted, is a conservative estimate — Siraj conducted 100 million
simulations, tweaking factors like orbit shape to determine how many rocky
worlds the sun may have captured. Siraj also assumed the sun's birth
environment was pretty cramped, to estimate the number in the worst-case
scenario, as under such circumstances "planetary capture is more
difficult," Siraj wrote in an email to Live Science.
The study found that
two planets with a Mars-like mass — or three to five with a mass similar to
Mercury's — may dwell roughly 1,400 astronomical units (AU) from the sun. (One
AU is the distance between Earth and the sun.) That would place the trapped rogue
planets in the Oort Cloud,
The newly proposed
planets are independent of Planet X,
a yet-to-be-discovered hypothetical Neptune-like world hypothesized to orbit
about 43 AU from the sun. Unlike the new study, which relies on theory,
scientists have predicted Planet X's existence based on observations of the
strange trajectories of objects in the Kuiper Belt, a doughnut-shaped region of
icy bodies that extends from Neptune to the Oort Cloud.
Even though the
captured terrestrial worlds would likely be more Earth-like than Planet X,
their suitability as habitable real estate remains speculative. While water may
exist as icy sheets, sunlight would be "very faint — comparable to the
brightness [of] moonlight here on the Earth," Siraj said. But because the
theorized planets come from other star systems, and are thus exoplanets,
future space missions investigating exoplanets may target them.
Yet, finding the newly proposed planets will be even trickier than identifying Planet X, as they're much smaller and more distant than that proposed planet. Siraj believes the under-construction Vera C Rubin Observatory in Chile, slated to go live in 2025, could identify a former FFP lying closer than 700 AU — but only, he noted, if "it's in the Southern sky and close enough to us, with a high enough albedo,” or ability to reflect light that illuminates it.
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