free-floating planet

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Free-floating planets are not gravitationally bound to any star. Their discovery was made possible by two methods: direct observation in the infrared if the planet is young and hot, and gravitational microlensing — when the planet focuses the light of a background star into a characteristic symmetric brightness spike. The light curve conveys information about mass and distance, and the absence of repeated events indicates its isolation.

History

The first reliable candidates for free-floating planets were discovered in 2004 from their faint infrared glow in the Orion cluster and from microlensing events.

How it works

Planets are born in gas and dust disks around stars. If another massive planet is nearby or a star flies by, gravitational slingshot can tear a smaller planet from its parent star's embrace, turning it into an eternal wanderer. To find it, astronomers look for "bumps" on light curves — a momentary amplification of light predicted by relativity.

💡 According to one estimate, there could be up to 20 billion free-floating planets in the Milky Way — more than stars — and some of them can hold onto moons.

A free-floating planet (also rogue planet) is a planetary-mass object not gravitationally bound to any star and drifting through the galaxy as an independent body. Such objects form in protoplanetary disks similarly to ordinary planets, but subsequent dynamical instability in the system (planet-planet scattering, tidal interaction with the star, external perturbation from a passing star) leads to their ejection. A less common channel is the collapse of an isolated gas-dust cloud, similar to star formation but at the limit of the smallest possible mass.

History

The first candidates were substellar-mass objects in the Orion cluster, recorded in deep infrared surveys in the late 1990s – early 2000s. A key breakthrough came from gravitational microlensing experiments (OGLE, MOA) starting in the 2000s, revealing many short, star-unassociated lensing events. In 2011, a statistically significant population of invisible planetary masses in the Galaxy was announced.

How it works

The main detection method is gravitational microlensing. When a planet passes near the line of sight to a background star, its gravitational field, according to general relativity, curves spacetime and focuses light, creating two images that are individually unresolved but produce a spike in total brightness. The characteristic angular scale is the Einstein radius: θ_E = √((4GM)/(c²) · (D_ls)/(D_l D_s)), where M is the lens mass, D_l, D_s, D_ls are distances (observer–lens, observer–source, lens–source). The event duration t_E = θ_E/μ_rel, where μ_rel is the relative proper motion, is limited to a few hours for Earth-mass planets. Free-floating planets produce simple, symmetric curves without additional caustics, distinguishing them from lensing campaigns for exoplanets in bound systems.

\theta_E = \sqrt{\frac{4GM}{c^2} \frac{D_{ls}}{D_l D_s}}

θ_E — angular Einstein radius (radians), G — gravitational constant (6.67430×10⁻¹¹ m³·kg⁻¹·s⁻²), M — lens mass (kg), c — speed of light (299792458 m/s), D_ls — distance from lens to source (m), D_l — distance from observer to lens (m), D_s — distance from observer to source (m)

Open problems & fun facts

Initial mass function: what is the contribution of isolated cloud collapse versus disk ejection?; Could such a planet have a subsurface ocean and sustain life through radioactive heat?; Distribution in the Galaxy and total number of Earth-mass objects

💡 Microlensing can detect planets as small as 0.01 Earth masses at distances to the Galactic center, with events lasting from hours to days.

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free-floating planet

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How it works

History

How it works

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How it works

Open problems & fun facts

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