How do you find a planet with no light?
Its own warmth
Young rogue planets are still hot from forming and glow faintly in infrared. Telescopes such as JWST and the VLT pick them out in young star clusters, where they are brightest.
A flicker of starlight
When an unseen planet passes in front of a distant star, its gravity bends and briefly brightens that star's light, for hours or less. This "microlensing" finds rogue planets far too dark to see at all.
Planet or failed star?
The line is blurry. Above about 13 Jupiters an object can burn a little deuterium and is usually called a brown dwarf. Below that, astronomers still argue whether a lone object formed like a planet or like a tiny star.
What comes next
NASA's Roman Space Telescope, due to launch by May 2027, is expected to find hundreds of rogue planets by microlensing, down to the mass of Mars.
Masses for directly seen objects come from their brightness and age, so they depend on models; microlensing masses usually depend on an unknown distance. Each card links to the discovery or key paper. The 3D views are artist's impressions from these numbers, not photos.