Ultracool dwarf

An ultra-cool dwarf is a stellar or sub-stellar object of spectral class M that has an effective temperature under 2,700 K (2,430 °C; 4,400 °F).TRAPPIST-1 is a widely known example of an ultra-cool dwarf star.

The category of ultra-cold dwarf stars was introduced in 1997 by J. Davy Kirkpatrick, Todd J. Henry, and Michael J. Irwin. Today it includes very low mass M-dwarf stars with spectral types of M7 and later, and extends beyond the coolest stars with spectral types near L4 to brown dwarfs. Together, they represent about 15% of astronomical objects in the stellar neighborhood of the Sun.

Models of the formation of planets suggest that due to their low masses and the small size of their proto-planetary disks, these stars could host a relatively abundant population of Earth-like planets ranging from Mercury-size to Earth-size instead of super-Earths and Jupiter-massed planets. The discovery of the TRAPPIST-1 system of seven Earth-like planets seems to validate this accretion model.

Due to their slow hydrogen fusion compared to other types of low mass stars their life spans are estimated at hundred billion years with the smallest living for trillions of years. As the age of the universe is only 13.8 billion years, all cool dwarf stars are relatively young. Models predict that at the ends of their lives the smallest of these stars will become blue dwarfs instead of expanding into red giants.

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