Confirmed exoplanet · Database record + computed analysis
ups And b
A confirmed world orbiting ups And, 43.7 light-years away.
A gas giant
14.00 Earth radii, Jupiter-class (Jupiter is 11.2). At only 218.5 Earth masses for that size, it must be substantially gas or volatile ices: puffy, not rocky. Orbiting this close, it is a classic 'hot Jupiter', a giant that migrated inward from where it formed.
The orbit
Its year, one full orbit, takes just 4.6 Earth days.
Temperature and starlight
Its equilibrium temperature (the airless-world estimate) is about 1,251 °C. Hot enough to melt copper; any surface would glow. It receives 956.0 times the starlight Earth gets.
Its sun
Its star is hotter than the Sun (6,157 K, F8 V), burning brighter and faster. The system holds 2 stars, so its skies have more than one sun. It shares the system with 2 other known planets.
How far is that, really?
At 44 light-years, tonight's light from this system left it around the year 1982. That is close by galactic standards, which makes it a prime target for follow-up telescopes. A Voyager-speed probe would need roughly 773,566 years to get there.
How we found it
It was found by the radial-velocity method: the planet's gravity swings its star in a small circle, and that wobble shows up as a rhythmic Doppler shift in the starlight. The wobble's size gives the planet's minimum mass. The discovery was announced in 1996 from Lick Observatory.
Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.
The deep dive
Researched for the Atlas from Wikipedia — Upsilon Andromedae b (6,018 characters read) · updated Sep 20, 2026
01 A Hot Jupiter Among the First Ever Found
When Geoffrey Marcy and R. Paul Butler announced the existence of Upsilon Andromedae b in January 1997, it joined a very short list of confirmed exoplanets and helped establish a brand-new class of world. The planet orbits its host star in just 4.617 days, completing a full year in roughly the time it takes Earth to get through a work week. Its semimajor axis is only 0.0595 AU, placing it far closer to its star than Mercury sits to our Sun. That extreme proximity made it one of the first confirmed hot Jupiters, a category that had seemed almost impossible before 51 Pegasi b upended expectations in 1995. The detection method was radial velocity: astronomers measured the tiny Doppler shifts in the star's spectrum caused by the gravitational tug of the orbiting planet. Announced alongside 55 Cancri b and the planet orbiting Tau Boötis, its discovery in June 1996 and public announcement in January 1997 marked a pivotal moment in the young science of exoplanet detection.
02 The Mass Problem: What We Still Do Not Know Deeper
Radial velocity detections carry a fundamental ambiguity: they reveal only the component of a planet's gravitational pull directed along our line of sight, so the derived mass is a lower limit that depends on the unknown orbital inclination. For Upsilon Andromedae b, analyses of the dynamic stability of the full planetary system and a preference for prograde orbital solutions constrain the inclination to somewhere between 5 and 23 degrees, implying a true mass ranging from 1.78 to 8.02 times Jupiter's mass. The preferred solution sits at an inclination of 14 degrees, pointing to a true mass of 2.81 Jupiter masses. A separate high-resolution spectroscopy study found 1.70 Jupiter masses at an inclination of 24 degrees, but those results were later called into doubt because the planet should not have been detectable with the data available. The uncertainty is not merely academic: a planet at the low end of that range behaves very differently in terms of atmospheric dynamics and interior structure than one at the high end, making this an open and practically important question.
03 A World of Extreme Temperature Contrasts
In 2006, observations from the Spitzer Space Telescope revealed something striking about Upsilon Andromedae b: the brightness of the entire star-planet system varied in step with the planet's orbit. That rhythmic brightening and dimming meant that light was coming from the planet itself, not just the star. By analyzing that phase curve, astronomers measured a temperature difference between the planet's two hemispheres of roughly 1,400 degrees Celsius. The perpetually star-facing dayside reaches somewhere between 1,400 and 1,650 degrees Celsius, while the permanent nightside languishes at somewhere between minus 20 and 230 degrees Celsius. Because the planet orbits so close and so fast, tidal forces likely lock its rotation so that one face always bakes and the other always freezes — a day that never ends on one side, a night that never lifts on the other. This measurement provided some of the earliest direct confirmation that hot Jupiters really do develop dramatically different dayside and nightside thermal environments.
04 Clouds of Iron and Silicate Above a Dark Shell Deeper
Astronomer David Sudarsky predicted the atmospheric structure of Upsilon Andromedae b by assuming its composition is broadly similar to Jupiter's and that its chemistry sits close to thermodynamic equilibrium. Under those conditions, the upper atmosphere should host reflective clouds made of silicates and iron condensates. However, rather than reflecting starlight back into space, this cloud deck absorbs the incoming radiation. Beneath it, sandwiched between the absorbing clouds above and the hot, high-pressure gas surrounding the planetary mantle below, lies a stratosphere of comparatively cooler gas. The outermost visible shell of cloud is thought to be dark and opaque, and the leading candidates for its composition are vanadium oxides and titanium oxides — compounds known to absorb visible light strongly. Other complex organic molecules called tholins cannot yet be ruled out. So far, the only molecule actually detected in the planet's atmosphere through absorption lines in its thermal infrared spectrum is water vapor. Carbon monoxide and methane remain below current detection limits.
05 An Enormous Planet With No Solid Ground
Given its high mass, Upsilon Andromedae b is expected to be a gas giant with no solid surface, much like Jupiter and Saturn in our own solar system. The Spitzer phase-curve data, combined with estimates of the orbital inclination, allow astronomers to estimate the planet's physical size. As of 2014, the best estimate places its radius at about 1.8 times Jupiter's radius. To put that in perspective, Jupiter itself is more than eleven times the diameter of Earth, so this world spans something close to twenty Earth diameters across. Its inflated size relative to Jupiter is consistent with the intense stellar irradiation it receives at its very close orbit, which can puff up gas giant atmospheres dramatically. The planet is also judged unlikely to host any large moons: tidal forces at such close range to the star would either fling moons out of stable orbit or pull them apart entirely on timescales far shorter than the age of the system, which is estimated at 3.12 billion years.
06 Magnetic Fingerprints on the Parent Star Deeper
One of the more surprising findings about Upsilon Andromedae b is its apparent influence on its host star's behavior. Observations reveal that the star shows elevated chromospheric activity — signs of magnetic heating in its outer layers — and crucially, this activity concentrates in a hot spot located roughly 169 degrees away from the point on the stellar surface closest to the planet. A hot spot offset from the sub-planetary point by that amount is difficult to explain by simple tidal heating alone. Instead, researchers suspect the planet's magnetic field is interacting directly with the star's magnetic field, channeling energy into specific regions of the stellar chromosphere. The proposed mechanism draws comparison to two known cases: the RS Canum Venaticorum class of binary stars, where closely orbiting stellar companions drive magnetic activity, and the electromagnetic interaction between Jupiter and its volcanic moon Io, where electric currents flow along magnetic field lines to create auroral hot spots on Jupiter itself. If confirmed, this would make Upsilon Andromedae b one of the clearest examples of a planet shaping its own star's magnetic behavior.
07 Direct Detection: Claims, Doubts, and Dead Ends Deeper
Upsilon Andromedae b was identified alongside 51 Pegasi b as a candidate for direct imaging by the Planetpol instrument, which was designed to detect polarized light from planets. The planet carries the additional distinction of being described as one of the first non-resolved planets to be detected directly — meaning light from the planet itself was captured without spatially separating it from the star in an image. In 2016 and 2017, researchers claimed to have directly detected the planet's thermal emission, a significant step beyond the Spitzer phase-curve result. However, that claimed detection was called into question in 2021. Separately, attempts to detect the planet using high-resolution interferometric observations with the CHARA array also failed to produce a confirmed signal. The history of these attempts illustrates how difficult direct detection remains even for a massive, hot, close-in planet around a relatively nearby star, and leaves the precise characterization of Upsilon Andromedae b as an ongoing challenge for the field.
08 The Star Titawin: Bright Enough to See Unaided
Upsilon Andromedae b orbits a star officially named Titawin, also catalogued as Upsilon Andromedae A, sitting about 44 light-years from the Sun in the constellation Andromeda. The star is an F-type main-sequence star, meaning it runs slightly hotter and more luminous than our own Sun. Its mass is 1.27 times the Sun's, its radius about 1.48 solar radii, and its surface temperature 6,074 Kelvin compared to the Sun's roughly 5,778 Kelvin. Its luminosity is 3.57 times that of the Sun. The star is estimated to be 3.12 billion years old, making it younger than the Sun's 4.6 billion years. It is also slightly metal-rich, with an iron abundance about 123 percent of the solar value. Perhaps most accessibly, Titawin shines at an apparent magnitude of 4.09, comfortably visible to the naked eye under dark skies — one of the few exoplanet host stars that any backyard observer can point to without a telescope.
09 Named for a Medieval Astronomer of Muslim Spain
For most of its known existence, Upsilon Andromedae b carried only its catalog designation. That changed through the International Astronomical Union's NameExoWorlds initiative, launched in July 2014 as a global public process inviting nominations and votes for official proper names for exoplanets and their stars. In December 2015, the IAU announced that this planet would officially be named Saffar. The winning name was submitted by the Vega Astronomy Club of Morocco. It honors Ibn al-Saffar, an eleventh-century astronomer who worked in Muslim Spain — a region and era that produced foundational contributions to mathematics, optics, and observational astronomy. The name connects a planet discovered at the cutting edge of late twentieth-century science to a scholarly tradition more than a millennium old, reminding us that the human drive to study the sky stretches far back before any modern telescope existed. The host star Upsilon Andromedae A received the companion name Titawin through the same process.
10 Where Saffar Sits in Its Planetary System
Saffar is the innermost known planet of the Upsilon Andromedae system, hugging its star at a distance of just 0.0595 AU and completing each orbit in 4.617 days. For comparison, Mercury, the closest planet to our Sun, orbits at roughly 0.39 AU — more than six times farther from the Sun than Saffar sits from Titawin. The system is known to host additional planets farther out, making Saffar the anchor of a multi-planet architecture. Its extreme closeness to the star means it experiences intense radiation and tidal forces that shape nearly every aspect of its physical character, from its inflated radius to its locked rotation and the dramatic temperature gradient between its two permanent hemispheres. Understanding how such a massive planet ended up so close to its star — almost certainly through a process of inward migration from a more distant formation location — remains one of the central questions that Saffar and its sibling hot Jupiters were discovered to pose.
11 What the Infrared Spectrum Can and Cannot Tell Us Deeper
Because Upsilon Andromedae b is so hot, its atmosphere radiates most strongly at infrared wavelengths, and it is in this part of the spectrum that astronomers search for the fingerprints of individual molecules through their absorption lines. The approach is challenging: the planet's signal is buried in the much brighter glare of its host star, requiring careful subtraction and high sensitivity. So far, the effort has successfully identified water vapor in the planet's atmosphere. Carbon monoxide and methane, both molecules commonly sought in hot Jupiter atmospheres, remain below the detection threshold with current instruments. The absence of a detection does not mean these molecules are absent — it may simply mean the signal is too faint or the mixing ratios too low for present technology to resolve. As telescope sensitivity improves, particularly with space-based infrared observatories and ground-based high-resolution spectrographs, the atmospheric inventory of Saffar is expected to grow, potentially revealing how its chemistry compares to other hot Jupiters with similar temperatures and irradiation levels.
About this record
The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.