Upsilon Andromedae

Lore & Background

The system's Bayer designation is Upsilon Andromedae, with components designated A and B. The primary star, Titawin, is a yellow-white dwarf younger and more massive than the Sun, with low X-ray emission suggesting it may be moving off the main sequence. The red dwarf companion, discovered in 2002 via 2MASS data, has a projected separation of 750 AU from the primary. The system also includes two other listed companions (components B and C in the Washington Double Star Catalog), though only the red dwarf is believed to be physically associated.

The innermost planet, Upsilon Andromedae b, was discovered in 1996 and announced in January 1997 by astronomers Geoffrey Marcy and R. Paul Butler. The two outer planets were independently confirmed in 1999 by teams at San Francisco State University and the Harvard-Smithsonian Center for Astrophysics. The planets are all comparable in size to Jupiter, with orbits that are not coplanar with each other or with the stellar rotation. Planet c has an orbit significantly inclined from the others, and the eccentricities of the outer planets may have arisen from a close encounter with a fourth planet that was subsequently ejected or destroyed.

In July 2014, the IAU launched the NameExoWorlds process, and in December 2015 the winning names—submitted by the Vega Astronomy Club of Morocco—were announced: Titawin for the star, and Saffar, Samh, and Majriti for the planets. The star's name derives from the Berber name for Morocco's Tétouan and Tunisia's Tataouine; the planets honor 10th- and 11th-century astronomers of Muslim Spain.

Reader's Guide

Upsilon Andromedae holds a foundational place in exoplanetary science as the first multiple-planet system discovered orbiting a main-sequence star and the first such system found in a multiple-star configuration. Its discovery in the late 1990s demonstrated that planetary systems could harbor multiple giant planets in diverse orbital configurations, challenging earlier models that assumed systems would resemble the Solar System's coplanar, near-circular orbits. The system's three jovian planets—with the innermost orbiting at just 0.05 AU and the outermost residing in the habitable zone—provided early evidence for planetary migration and dynamical interactions. The non-coplanar orbits, particularly the inclined orbit of planet c, suggested that gravitational perturbations from the companion star or from a now-lost fourth planet could shape system architecture. The system also contributed to the development of radial velocity detection methods, as the innermost planet's large wobble made it one of the first exoplanets detected via this technique. In 2015, the IAU's NameExoWorlds campaign gave the system culturally significant names rooted in Berber and Islamic astronomical heritage, linking modern discovery to historical scholarship. The star was ranked 21st on NASA's target list for the canceled Terrestrial Planet Finder mission, underscoring its continued relevance for future studies of planetary habitability and system evolution.

A Landmark in Multiplanetary Discovery

The identification of Upsilon Andromedae d in 1999 represented a watershed moment in exoplanet science. At the time, astronomers already knew the hot Jupiter Upsilon Andromedae b circled the host star, but the residual wobble in the star's radial velocity curve could not be accounted for by that single world alone. Researchers at San Francisco State University and the Harvard-Smithsonian Center for Astrophysics independently arrived at the same conclusion: a three-planet architecture best explained the observed Doppler shifts in the star's spectrum. This made the Upsilon Andromedae system the first confirmed multiplanetary system around a main-sequence star, and the first such system known within a multiple-star configuration. The detection relied on the radial velocity technique, in which the gravitational tug of an orbiting body produces periodic shifts in spectral lines. A key early limitation was that this method yields only a minimum mass, initially estimated at roughly 4.1 Jupiter masses. It was not until ground-based radial velocity data were merged with astrometric observations from the Hubble Space Telescope that the true mass of 10.25 Jupiter masses and the orbital inclination of 23.8 degrees were firmly established.

A Super-Jupiter Around a Slightly Evolved Sun

Upsilon Andromedae d is classified as a super-Jupiter, carrying a mass of 10.25 times that of Jupiter with a likely radius of roughly 1.02 Jupiter radii. Its equilibrium temperature sits at 218 kelvin, equivalent to about minus 55 degrees Celsius. The planet circles Upsilon Andromedae A, an F-type star that is somewhat more massive and larger than our Sun, with a mass of 1.27 solar masses and a radius of approximately 1.48 solar radii. The star's surface temperature of 6,074 kelvin exceeds the Sun's 5,778 kelvin, and its luminosity reaches 3.57 times solar output. At 3.12 billion years of age, the host star is noticeably younger than the Sun's 4.6 billion years. The star is slightly metal-rich, with an iron-to-hydrogen ratio of 0.09, corresponding to about 123 percent of the Sun's metal content. From Earth, the system lies roughly 44 light-years away in the constellation Andromeda, and the star's apparent magnitude of 4.09 means it can be spotted with the unaided eye under dark skies.

An Eccentric Orbit and a Possible Lost World

Upsilon Andromedae d completes one orbit around its host star in approximately 1,276 days, or about 3.5 years. What makes this orbit remarkable is its pronounced eccentricity; it is more elongated than the path of any planet in our own Solar System. One leading hypothesis to explain this unusual shape invokes a gravitational close encounter with an outer planet that no longer exists. In that scenario, the interaction would have kicked planet d into a tighter, more eccentric trajectory while flinging the outer companion out of the system entirely. The mutual inclination between the orbits of planets c and d has been measured at 29.9 degrees. Early astrometric measurements hinted at an inclination of 155.5 degrees relative to the plane of the sky, but those figures ultimately proved useful only as upper limits and conflicted with the known inclination of the inner planet. The definitive inclination of 23.8 degrees emerged only after combining Hubble Space Telescope astrometry with ground-based radial velocity data, resolving a long-standing ambiguity in the system's geometry.

Majriti: A Name and a Hope for Distant Moons

In July 2014, the International Astronomical Union launched NameExoWorlds, an initiative inviting the public to propose and vote on official names for select exoplanets and their host stars. In December 2015, the IAU announced that Upsilon Andromedae d would carry the name Majriti, a submission from the Vega Astronomy Club of Morocco honoring the 10th-century Andalusian scholar Maslama al-Majriti. Beyond its name, the planet sparks genuine scientific curiosity about habitability. Although Majriti itself is a gas giant and almost certainly cannot support life, its position within the habitable zone of Upsilon Andromedae A, defined by the capacity for liquid water to persist and by the ultraviolet flux received, raises the possibility that a large moon or moons could be temperate worlds. For a satellite to remain gravitationally bound to the planet, its orbital period must stay below roughly one-ninth of the planet's own year. In this system, that ceiling works out to about 120 days, or four months, for a stable lunar orbit.