47 Ursae Majoris

Lore & Background

47 Ursae Majoris is a G1V yellow dwarf star on the main sequence, converting hydrogen to helium in its core. It is slightly hotter than the Sun at around 5,880 K and has a similar mass, but its visual luminosity is about 60% greater. The star is slightly more metal-rich than the Sun, with about 105% of the solar abundance of iron. Its age is uncertain: chromospheric activity suggests around six billion years, while evolutionary models indicate an older age of about 8.7 billion years; other estimates range from 4.4 to 7 billion years. The low level of magnetic activity makes it a Maunder Minimum candidate star.

The star's Flamsteed designation is 47 Ursae Majoris. In July 2014, the International Astronomical Union launched the NameExoWorlds process, and in December 2015 the winning names were announced: Chalawan for the star, and Taphao Thong and Taphao Kaew for two of its planets. The names were submitted by the Thai Astronomical Society, Thailand. Chalawan is a mythological crocodile king from the Thai folktale Krai Thong, and Taphao Thong and Taphao Kaew are two sisters associated with the tale. The name Chalawan also refers to an extinct genus of crocodilian containing a single species, Chalawan thailandicus. In 2016, the IAU's Working Group on Star Names recognized the names approved during the 2015 campaign, and the star is now entered in the IAU Catalog of Star Names.

Reader's Guide

47 Ursae Majoris is significant as the host of the first long-period extrasolar planet discovered, announced in 1996 by Geoffrey Marcy and R. Paul Butler. That planet, 47 Ursae Majoris b (Taphao Thong), has a low-eccentricity orbit and a minimum mass 2.53 times that of Jupiter, taking 1,078 days to orbit. A second planet, 47 Ursae Majoris c (Taphao Kaew), was announced in 2002 by Debra Fischer, Geoffrey Marcy and R. Paul Butler, with an orbital period around 2,391 days, though subsequent measurements left its parameters uncertain and a longer dataset suggested a period near 7,586 days. A third planet, 47 Ursae Majoris d, was discovered in 2010 using the Bayesian Kepler Periodogram, with an orbital period of 14,002 days and a semi-major axis of 11.6 AU. The system's configuration has been compared to Jupiter and Saturn in the Solar System.

The star was listed among the top 100 target stars for NASA's former Terrestrial Planet Finder mission and among 164 target stars for the Habitable Worlds Observatory. Simulations suggest the inner part of the habitable zone could host a stable terrestrial planet, though the outer regions would be disrupted by the giant planet 47 UMa b. Two METI messages have been sent to the star: the Teen Age Message in 2001 and Cosmic Call 2 in 2003, both from the 70 m Eupatoria Planetary Radar, with arrival dates in 2047 and 2049 respectively.

A Solar Analog in the Northern Sky

47 Ursae Majoris, formally known as Chalawan, sits roughly 45.3 light-years from Earth in the constellation Ursa Major. Classified as a G1V yellow dwarf, it is a close cousin of our Sun—comparable in mass but running a few hundred kelvin hotter at approximately 5,880 K. Its visual luminosity exceeds the Sun's by about 60%, giving it an absolute magnitude of +4.29, while its apparent magnitude of +5.03 keeps it just within naked-eye visibility. The star carries a slightly elevated iron abundance, roughly 105% of solar metallicity. Like the Sun, it fuses hydrogen into helium in its core along the main sequence. Estimating its age proves tricky: chromospheric activity points to roughly six billion years, while evolutionary models push it closer to 8.7 billion, with other studies landing at 4.4 or 7 billion. Its remarkably low magnetic activity has led researchers to flag it as a potential Maunder Minimum candidate—a star in a prolonged quiet phase analogous to the famous solar minimum of the 17th century.

A Name from Thai Mythology

In July 2014, the International Astronomical Union launched NameExoWorlds, an open campaign inviting the public to propose and vote on proper names for select exoplanet systems. The Thai Astronomical Society entered the contest with a submission drawn from Thai folklore. In December 2015, the IAU announced the winners: the star received the name Chalawan, a mythological crocodile king from the Thai folktale Krai Thong, while the two inner planets were christened Taphao Thong and Taphao Kaew, the two sisters associated with that same tale. The name Chalawan also happens to be the designation of an extinct crocodilian genus, Chalawan thailandicus. In 2016, the IAU's Working Group on Star Names formally cataloged these approved names in its first bulletin, entering them into the IAU Catalog of Star Names alongside the Flamsteed designation 47 Ursae Majoris. The campaign thus gave a small, nearby star a cultural identity rooted in Southeast Asian storytelling, a first for many observers who had only known it by its catalog number.

Unveiling a Three-Planet System

The first planet around 47 Ursae Majoris was announced in 1996 by Geoffrey Marcy and R. Paul Butler, detected through the Doppler shift of the star's spectrum as the planet's gravity tugged it into a wobble. At a minimum mass of 2.53 Jupiter masses and an orbital period of about 1,078 days, 47 UMa b was the first long-period extrasolar planet ever found, tracing a low-eccentricity path that would sit between Mars and Jupiter if transplanted into our own system. A second candidate, 47 UMa c, was reported in 2002 by Debra Fischer, Marcy, and Butler using the same radial-velocity technique, with an orbital period near 2,391 days. Yet follow-up measurements struggled to confirm it, and longer datasets shifted the best-fit period to roughly 7,586 days, leaving its parameters highly uncertain. The third planet, 47 UMa d, emerged in 2010 through Bayesian Kepler Periodogram analysis by Fischer and P. C. Gregory. At 1.64 Jupiter masses and a 38.33-year orbit at 11.6 AU, it was then the longest-period planet found via radial velocity. The three-planet model proved 100,000 times more probable than a two-planet one.

The Question of Terrestrial Worlds

Simulations of the 47 Ursae Majoris system suggest that a terrestrial planet could maintain a stable orbit in the inner portion of the star's habitable zone, though the outer edge of that zone would be gravitationally perturbed by 47 UMa b. The presence of a giant planet within roughly 2.5 AU of the star raises a further concern: it may have disrupted the planet-formation process in the inner disk and curtailed the delivery of water to any nascent rocky worlds during the accretion phase. This combination of gravitational instability and reduced volatile transport casts doubt on whether the habitable zone of Chalawan could ever support a truly Earth-like planet with a substantial water inventory. Meanwhile, the orbital architecture of the three known giants—spanning periods from under three years to over three decades—creates a complex gravitational landscape. Further refinements of the planets' orbits in the 2020s have sharpened our picture, but the fundamental question of whether the inner system remained pristine enough to nurture life-friendly conditions remains open. The star's proximity, at just 45 light-years, makes it a compelling target for future high-precision observations that may finally resolve these uncertainties.