Teegarden’s Star

Teegarden’s Star (SO J025300.5+165258) is a red dwarf located 12.497 light-years away in the constellation Aries. With an apparent magnitude of 15.14, it is invisible to the unaided eye. It is the 31st nearest star to the Sun.

Teegarden’s Star hosts a system of at least three orbiting planets. The planetary system was discovered in 2019. The two inner planets, Teegarden’s Star b and c, are potentially habitable. They are among the most Earth-like extrasolar planets known.

Star type

Teegarden’s Star is a dim red dwarf of the spectral type M7.0 V. Even though it is one of the nearest stars to Earth, with an apparent magnitude of 15.14, it can only be seen in larger telescopes. The star is much older than the Sun, with an estimated age of over 8 billion years.

The small red dwarf has a mass of 0.097 solar masses and a radius of 0.120 solar radii. With an effective temperature of around 3,034 K, it shines with only 0.000722 solar luminosities. A mass only 9.7% that of the Sun is close to the limit between a true star and a brown dwarf. Most of the red dwarf’s energy output is in the infrared part of the spectrum.

The star is a slow spinner. With a projected rotational velocity of less than 2 km/s, it takes 98.05 days to complete a rotation.

Like many other nearby red dwarfs, Teegarden’s Star exhibits stellar flares. However, it only shows low levels of magnetic activity for an M-dwarf, like Ross 128 in the constellation Virgo and Luyten’s Star in Canis Minor.

Teegarden’s Star hosts one of the nearest planetary systems to the Sun. The two inner planets were discovered in 2019, and the outer planet was reported in 2024. All three were detected using the radial velocity method, by observing the effects of the planets on the radial velocity of their red dwarf host. Observations with the Transiting Exoplanet Survey Satellite (TESS) have confirmed that the planets do not transit Teegarden’s Star from our point of view.

Teegarden’s Star b and c are among the nearest potentially habitable planets to Earth, along with those orbiting the red dwarfs Proxima Centauri, Ross 128, Gliese 1061, Wolf 1061 and Luyten’s Star, and the yellow dwarf Tau Ceti.

Artist’s conception of Teegarden’s Star (SO25300.5+165258), image credit: NASA/Walt Feimer (PD)

Planets

Teegarden’s Star hosts three known extrasolar planets, designated Teegarden’s Star b, c, and d (or Teegarden b, c, and d). Teegarden b is a super Earth exoplanet that orbits inside the star’s optimistic habitable zone, comparable to the zone between Venus and Earth in the solar system. Teegarden c orbits at the outer edge of the star’s conservative habitable zone, like Mars in our system. Teegarden d is farther out, beyond the goldilocks zone, and too cold to harbour liquid water.

The short-period planets Teegarden’s Star b and c were discovered by a team of more 50 scientists led by Dr. Mathias Zechmeister of the Institute for Astrophysics and Geophysics, University of Göttingen, in 2019. The team conducted the CARMENES survey (Calar Alto high-Resolution search for M-dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) at the Calar Alto Observatory in Spain and obtained more than 200 radial velocity measurements of Teegarden’s Star. The study announcing the discovery was published in the journal Astronomy & Astrophysics.

After analyzing the data for planetary signals, the astronomers found evidence for two planets, each with a mass of at least 1.1 Earth masses, with orbital periods of 4.91 and 11.4 days. Both planets are orbiting in the star’s habitable zone. If they have atmospheres, they could be hospitable to life.

At the time of discovery, Teegarden b and c were among the lowest-mass extrasolar planets known. They were also the only known planets orbiting a star with a surface temperature cooler than 3,000 K, except for the planets orbiting the low-mass stars Proxima Centauri and TRAPPIST-1. While Teegarden’s Star is now believed to have a temperature slightly higher than 3,000 K, it is one of the few faint, late-type stars with confirmed planetary systems.

Teegarden’s Star d was discovered by a team of astronomers led by S. Dreizler, Institute for Astrophysics and Geophysics, University of Göttingen, in 2024. The team reported the detection of a planet with an orbital period of 26.13 ± 0.04 days and a minimum mass of 0.82 ± 0.17 Earth masses. Radial velocity measurements with multiple instruments and TESS data allowed the researchers to refine the planetary and orbital parameters of Teegarden b and c, as well as the stellar parameters of Teegarden’s Star.

All three planets are much closer to their host star than Earth is to the Sun, and they receive more X-ray radiation than our planet. Teegarden b and Teegarden c orbit at distances where they may harbour liquid water.

Teegarden’s Star may host additional planets. Two other radial velocity signals have been reported. One of these (a 96-day signal) corresponds to the stellar rotation period of Teegarden’s Star. The origin of the other signal (a 172-day signal) is still unknown. It may be attributed to stellar activity, or it may indicate the presence of a long-period planet with a mass of at least 2.3 Earth masses. Another signal with a period of 7.7 days was identified in the CARMENES data, but not in other observations. It may correspond to a small planet with half the Earth’s mass.

Size comparison of the three confirmed planets of Teegarden’s Star (artistic concept) with Earth, calculated assuming Earth-like composition. Credit: Martin Vargic / Halcyon Maps (CC BY 3.0)

Teegarden’s Star b

Teegarden b is the innermost planet orbiting Teegarden’s Star. It has a mass of at least 1.16 Earth masses and orbits Teegarden’s Star with a period of 4.90634 ± 0.00041 days at a distance of 0.0259 astronomical units (Earth – Sun distances). The planet is believed to be rocky, with a radius of around 1.05 Earth radii.

The planet exhibits Earth-like characteristics. It has an instellation (amount of stellar radiation) of (1.08 S_⊙) and an equilibrium temperature of 277 kelvin (3.85° C) assuming an albedo of 0.3 (the fraction of reflected starlight similar to Earth’s).

Teegarden’s Star b was initially given an ESI (Earth Similarity Index) ranking of 0.97, the highest ESI ranking of any planet in the habitable planet catalogue. This was reduced to 0.90 following the observations that led to the discovery of Teegarden d.

A high ESI index does not indicate habitability because it does not take into account stellar activity, tidal locking and the planet’s magnetic field. Astronomers estimate that Teegarden b has a 60% chance of having a temperate surface environment, with a temperature of around 28° C, but that the odds of it having an atmosphere are only 3%.

Artist impression of Teegarden’s Star b. Made from the exoplanet catalog at NASA, hue changed to green to make it more in line with it being the most Earth-like exoplanet. Credit: NASA, Dhrm77 (PD)

Teegarden’s Star c

Teegarden c has an orbital period of 11.416 ± 0.003 days and an orbital distance of 0.0455 AU. It has an estimated mass of at least 1.05 Earth masses and an equilibrium temperature of around -47° C, similar to that of Mars. Our red planet has an average temperature of -63° C on average, but the temperature varies greatly between day and night. Daytime temperatures can be as high as 20° C, while nighttime temperatures can drop as low as -140° C.

If Teegarden c has an atmosphere, its surface would likely be much warmer than -47° C. In comparison, the Earth has an equilibrium temperature of -18° C (0° F), but the surface is warmer because the planet has an atmosphere.

Teegarden c has an Earth Similarity Index of 0.8, similar to Proxima b in the Proxima Centauri system. Like Teegarden b, it is believed to be a rocky planet with an iron core. It may have an ocean or ice on its surface.

Teegarden’s Star d

Teegarden d is the least massive of the three planets in the Teegarden’s Star system. It has a minimum mass of 0.82 Earth masses. It orbits the parent star with a period of 26.13 days at a distance of 0.0791 AU.

The planet was detected in 2024 using data obtained with the CARMENES instrument at Calar Alto Observatory, the ESPRESSO instrument (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations) at the Very Large Telescope (VLT), MAROON-X (M-dwarf Advanced Radial velocity Observer Of Neighboring eXoplanets) at the Gemini North telescope, and HPF (Habitable Zone Planet Finder) at McDonald Observatory. The radial velocity measurements were combined with photometric data obtained with TESS (the Transiting Exoplanet Survey Satellite).

Unlike the two inner planets, Teegarden d is cold, with temperatures resembling those of the planet Jupiter or its moon Ganymede. It orbits outside the habitable zone of Teegarden’s Star.

Facts

Teegarden’s Star was discovered by a team led by astrophysicist Bonnard J. Teegarden at the Laboratory for High Energy Astrophysics, NASA’s Goddard Space Flight Center, in 2003. The team reported the discovery of a nearby high proper motion star, SO025300.5+165258, while searching the SkyMorph database, which contained data obtained in the Near Earth Asteroid Tracking (NEAT) survey with two 1 m telescopes on Maui, Hawaii in the late 1990s.

The astronomers reported a proper motion of 5.06 +/- 0.03 arcseconds per year for the star. The study was published in The Astrophysical Journal. The star was later identified on photographic plates obtained during the Palomar Observatory Sky Survey in 1951.

Teegarden’s team measured a parallax of 0.43 ± 0.13 arcseconds for the star, which would have placed it 7.50 light years away. At this distance, Teegarden’s Star would be the third nearest star system to Earth, after Alpha Centauri and Barnard’s Star. It would be closer to us than Wolf 359, Lalande 21185 and Sirius.

In 2009, American astronomer George Gatewood found a parallax of 0.2593 seconds of arc for Teegarden’s Star, corresponding to a distance of 12.578 light-years. This is much closer to the currently accepted value of 12.497 light-years.

Teegarden’s Star is one of the very few stars with a proper motion of more than 5 arcseconds per year. Others include Barnard’s Star in the constellation Ophiuchus, Kapteyn’s Star in Pictor, Argelander’s Star (Groombridge 1830) in Ursa Major, Lacaille 9352 in Piscis Austrinus, Gliese 1 in Sculptor, and Bessel’s Star (61 Cygni) in Cygnus.

Teegarden’s Star is the 22nd nearest star system and the 31st closest individual star to the Sun. It lies at a similar distance to the red dwarf YZ Ceti (12.122 ly) in the constellation Cetus, Luyten’s Star in Canis Minor (12.348 ly), and Kapteyn’s Star (12.8308) in Pictor.

The nearest neighbours of Teegarden’s Star include the red dwarf TZ Arietis (L 1159-16) in the constellation Aries, which lies around 4 light-years from the star, the orange dwarf Epsilon Eridani in Eridanus (~5.8 ly), the yellow-white main sequence star HIP 15689 in Eridanus (~7.2 ly), the yellow dwarf Tau Ceti in Cetus (~7.7 ly), and the white dwarf known as Van Maanen’s Star in Pisces (~7.8 ly).

Name

Teegarden’s Star is named after the astrophysicist Bonnard J. Teegarden, who led the team that discovered it in 2003. The name has not been formally approved by the International Astronomical Union (IAU).

Teegarden’s Star is listed as GJ 10393 in the Gliese Catalogue of Nearby Stars and LSPM J0253+1652 in Lépine and Shara’s catalogue of high proper motion stars.

Location

Teegarden’s Star lies in the faint region of the sky between the constellation figure of Aries and the circle of stars that form the head of Cetus. At declination +17, it is theoretically visible from virtually anywhere on Earth. However, with an apparent magnitude of 15.14, the star can only be observed in large telescopes.

Teegarden’s Star location, image: Stellarium

Constellation

Teegarden’s Star is located in the zodiac constellation of Aries. The celestial Ram is one of the 48 Greek constellations, catalogued by Ptolemy of Alexandria in his Almagest in the 2nd century CE. In Greek mythology, it is associated with the golden ram that saved Phrixus and Helle, the children of King Athamas, and carried Phrixus to safety to Colchis. In Greek lore, Helle did not survive the journey. She fell off the ram’s back and plummeted into a strait that was named Hellespont (“sea of Helle”) in her honour. The strait is now known as the Dardanelles.

With only two stars brighter than magnitude 3.0, Aries is not particularly conspicuous. However, it can be easily made out on a clear night because its three bright stars – Hamal (Alpha Arietis), Sheratan (Beta Arietis) and Mesarthim (Gamma Arietis) – form a flat triangle that appears just below the elongated triangle of Triangulum.

Hamal, the brightest star in Aries, shines at magnitude 2.0 from a distance of 65.8 light-years. The K-type red giant marks the Ram’s forehead, while the fainter Sheratan and Mesarthim appear in one of its horns. Sheratan is an A-type star in a binary system and the fainter Mesarthim is a chemically peculiar star in a triple star system.

Other relatively bright stars in Aries include the triple star 41 Arietis (Bharani) and the K-type giants Lilii Borea (39 Arietis), HD 20644, and Botein (Delta Arietis).

Aries also hosts the orange dwarf HD 12661 and yellow dwarf HD 17674, each with two orbiting planets, the yellow dwarf HIP 14810 with three planets, the nearby red dwarf TZ Arietis (L 1159-16) with at least one exoplanet, and the magnetic, chemically peculiar star 56 Arietis (SX Arietis), a prototype for a class of variable stars known as the SX Arietis variables.

Aries constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

Deep sky objects in Aries include the spiral galaxies NGC 772 (the Fiddlehead Galaxy), NGC 691, NGC 972, NGC 695, IC 167, the elliptical galaxies NGC 770, NGC 821 and NGC 680, and the interacting galaxies Arp 276 (NGC 935 and IC 1801) and UGC 2369.

The best time of the year to observe the stars and deep sky objects in Aries is during the month of December, when the constellation appears higher above the horizon in the early evening. The entire constellation is visible from locations north of the latitude 60° S.

The 10 brightest stars in Aries are Hamal (Alpha Ari, mag. 2.00), Sheratan (Beta Ari, mag. 2.66), Bharani (41 Ari, mag. 3.63), Mesarthim (Gamma Ari, mag. 3.86), Botein (Delta Ari, mag. 4.349), HD 20644 (mag. 4.47), Lilii Borea (39 Ari, mag. 4.514), 35 Arietis (mag. 4.64), Lambda Arietis (mag. 4.79), and Zeta Arietis (mag. 4.89).

Teegarden’s Star