Luyten b (more commonly known as Gliese 273b) is a confirmed

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

, likely rocky, orbiting within the

habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...

of the nearby

red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...

Luyten's Star Luyten's Star (GJ 273) is a red dwarf in the constellation Canis Minor located at a distance of approximately from the Sun. It has a visual magnitude of 9.9, making it too faint to be viewed with the unaided eye. It is named after ...

. It is one of the most Earth-like planets ever found and is the fourth-closest potentially habitable exoplanet known, at a distance of 12

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s. Only

Proxima Centauri b Proxima Centauri b (or Proxima b), sometimes referred to as Alpha Centauri Cb, is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the triple star system Alpha ...

Ross 128 b Ross 128 b is a confirmed Earth-sized exoplanet, likely rocky, orbiting within the inner habitable zone of the red dwarf Ross 128, at a distance of about 11 light-years from Earth. The exoplanet was found using a decade's worth of radial veloci ...

, and

Gliese 1061 d Gliese 1061 is a red dwarf star located approximately from Earth in the southern constellation of Horologium. Even though it is a relatively nearby star, it has an apparent visual magnitude of about 13, so it can only be seen with at l ...

are closer. Discovered alongside Gliese 273c in June 2017, Luyten b is a

Super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...

of around 2.89 times the mass of Earth and receives only 6% more starlight than Earth, making it one of the best candidates for habitability. In October 2017 and 2018, the nonprofit organization METI ( Messaging Extraterrestrial Intelligence) sent a message containing dozens of short musical compositions and a scientific "tutorial" towards the planet in hopes of contacting any potential extraterrestrial civilizations.

Characteristics

Mass, radius, and temperature

Mass and size

Luyten b is a Super-Earth, meaning that it has a mass and/or radius greater than that of Earth, but less than that of

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

. Radial velocity measurements shows that the planet has a minimum mass of 2.89 , placing it at the lower end of the Super-Earth range. The planet has not been found to transit its star, and as a result its true mass and radius are not known. For an Earth-like composition, Luyten b would have to be about 1.4 . Due to its low mass, the planet is likely terrestrial, with a radius below 1.5 . In addition, it is one of the least massive planets found within the nearest 20 light-years.

Temperature and atmosphere

The planet receives an incident flux only 6% greater than that of Earth. With an estimated albedo, or proportion of light reflected by the planet, of 0.30, Luyten b has an equilibrium temperature of 259 °K. For comparison, Earth has an equilibrium temperature of 255 °K. With an Earth-like atmosphere — if it has one — Luyten b would have an average surface temperature of about , very similar to that of Earth.

Orbit and rotation

Luyten b orbits quite close to its host star. One full revolution around Luyten's Star takes about 18.6 days at an average distance of 0.091 AU, much closer in than

Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

, which has a year of 88 days and an orbital radius of 0.387 AU. However, because the host star is so dim, Luyten b falls right within the system's habitable zone and only receives 6% more starlight than Earth. While most planets this close to their parent star would be tidally locked, Luyten b has a moderately high orbital eccentricity of which may allow the planet to be captured in a

spin-orbit resonance Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...

, where the planet's rotation and orbital period can be simplified down to small integers (3:2, 1:2).

Host star

Luyten's Star is a medium-sized red dwarf star on the main sequence. It has 29.3% the radius, 29% the mass, 0.88% the luminosity of the Sun, and has an effective temperature of 3,382 °K. Unlike many nearby red dwarfs, like

Proxima Centauri Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...

, Luyten's Star is very inactive with a long rotation period of over 118 days.

Habitability

Similarity to Earth

Luyten b is one of the most Earth-like planets ever found. The first factor in its potential habitability is its distance from the host star. With an orbital radius of about 0.0911 AU, and given the star's low luminosity, Luyten b is orbiting well within the habitable zone. This is the region around a star where temperatures are just right for liquid water to pool on a planet's surface, given sufficient atmospheric pressure. Many other factors are also in favor of Luyten b's potential habitability. It's only about 2.89 times the mass of Earth, almost certainly making it a rocky planet. The gray zone between rocky and gaseous planets is believed to be around 6–8 , in which planets above 8 earth would be more akin to mini Neptune(s). Luyten b's minimum mass falls well below the 6–8 transition, and the chances for its actual mass to exceed the lower limit of the gray zone are only around 12%.

Temperature

In addition to its likely terrestrial nature, Luyten b is quite close in temperature to Earth, with an of 259 K. The planet is exposed to a stellar incident flux of 1.06 times that of Earth – not enough to cause a

runaway greenhouse effect A runaway greenhouse effect occurs when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A ...

: That would take more than 1.5 . While temperatures would probably be much higher on the direct star-facing side, a thick enough atmosphere would be able to distribute heat around the planet, providing life-friendly conditions to more of the planet. If Luyten b is found to be in a spin-orbit resonance with a rotation period less than or close to its orbital period, then this global heat distribution effect would be more efficient.

Interactions with host star

Unlike many other potentially habitable exoplanets orbiting red dwarfs, like Proxima b and the TRAPPIST-1 planets, Luyten b has the advantage of orbiting a very quiet host. Luyten's Star has a very long rotational period of 118 days and is not prone to powerful solar flares. Strong enough flare events can strip the atmospheres of orbiting planets and eliminate their chances of habitability; a good example of this is

Kepler-438b Kepler-438b (also known by its Kepler Object of Interest designation ''KOI-3284.01'') is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 472.9 light ...

. However, with the low activity of its host, Luyten b is likely to retain any atmosphere for billions of years, potentially enabling the development of life as we know it.

References

{{DEFAULTSORT:Luyten b Exoplanets discovered in 2017 Super-Earths in the habitable zone Canis Minor