An ultra-cool dwarf is a stellar or sub-stellar object that has an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

lower than . This category of

dwarf star A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars. The meaning of the word "dwarf" was later extended to some star-sized objects that are not stars, and compact stellar remnants that are ...

s was introduced in 1997 by J. Davy Kirkpatrick, Todd J. Henry, and Michael J. Irwin. It originally included very low mass M-dwarf stars with spectral types of M7 but was later expanded to encompass stars ranging from the coldest known to

brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

s as cool as spectral type T6.5. Altogether, ultra-cool dwarfs represent about 15% of the astronomical objects in the stellar neighborhood of the Sun. One of the best known examples is

TRAPPIST-1 , - ! style="background-color: #FFFFC0; text-align: center;" colspan="2", Characteristics , - style="vertical-align:top" ! style="text-align:left" , Evolutionary stage , Main sequence , - style="vertical-align:top" ! style="text-ali ...

. 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

terrestrial planet A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to ...

s ranging from Mercury-sized to Earth-sized bodies, rather than a population of

super-Earths A super-Earth is a type of exoplanet with a mass higher than Earth, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to t ...

and Jupiter-massed planets. The discovery of the TRAPPIST-1 planetary system, consisting of seven Earth-sized planets, would appear to validate this accretion model. Due to their slow

hydrogen fusion In astrophysics, stellar nucleosynthesis is the creation of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...

, when compared to other types of low-mass stars the life spans of ultra-cool dwarfs are estimated to be at least several hundred billion years, with the smallest among them living for about 12 ''trillion'' years. As the

age of the universe In physical cosmology, the age of the universe is the cosmological time, time elapsed since the Big Bang: 13.79 billion years. Astronomers have two different approaches to determine the age of the universe. One is based on a particle physics ...

is only 13.8 ''billion'' years, all ultra-cool dwarf stars are therefore in the early portions of their life-cycles. Models predict that at the ends of their lives the smallest of these stars will become blue dwarfs rather than expanding into

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

Magnetic properties

After the detection of bursts of radio emission from the M9 ultracool dwarf LP in 2001, a number of astrophysicists began observation campaigns at the

Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...

and the

Very Large Array The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory in the southwestern United States built in the 1970s. It lies in central New Mexico on the Plains of San Agustin, between the towns of Magdalena, Ne ...

to search for additional objects emitting radio waves. To date hundreds of ultra-cool dwarfs have been observed with these radio telescopes and of these stars, more than a dozen radio-emitting ultra-cool dwarfs have been identified. These surveys indicate that approximately 5-10% of ultracool dwarfs emit radio waves. These observation campaigns identified the noteworthy 2MASS J10475385+2124234, which has a temperature of 800-900 K making it the coolest known radio-emitting

(as of 2012). 2MASS J10475385+2124234 is a T6.5 brown dwarf that retains a

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

with a strength greater than 1.7 kG, making it some 3000 times more intense than

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...

. More recent observations found an even colder ultracool dwarf with radio emission, called WISEPA J062309.94-045624.6 (T8), with a temperature of around 740 K.

References

External links

UltracoolSheet
Catalog containing over 3,000 ultracool dwarfs and directly imaged exoplanets

Predecessor of the UltracoolSheet {{Exoplanet + *