The Star Fomalhaut: The Mouth of the Fish

A bright and isolated star in the southern sky

Fomalhaut, or Alpha Piscis Austrini, is the brightest star in the constellation of the Southern Fish (Piscis Austrinus). Visible to the naked eye from the temperate latitudes of the southern hemisphere and marginally in the north, it symbolically marks the "mouth of the fish" in the night sky. Located just 25 light-years from Earth, this A3 V spectral type star is about 16 times more luminous than the Sun and twice as massive. Its surface temperature is around 8,590 K, giving it a bright white-blue hue. Unlike many bright stars in the sky, Fomalhaut is relatively isolated: it stands alone in a sparsely populated region of the sky, making it a notable celestial landmark for ancient navigators and modern astronomers.

A planetary system in formation?

Since the 2000s, Fomalhaut has been scrutinized for its intriguing circumstellar environment. It is surrounded by a vast ellipsoidal debris disk extending over more than 140 AU (astronomical units), suggesting the presence of a young planetary system in formation. In 2008, the Hubble space telescope detected an object named Fomalhaut b, interpreted as an exoplanet. However, this optical detection without infrared confirmation has raised skepticism: the object could rather be an expanding dust cloud after a collision. More recently, in 2023, infrared data revealed other internal structures in the disk, suggesting that several massive bodies (planets or planetary embryos) are gravitationally interacting with the disk. Fomalhaut would thus represent a natural laboratory for understanding the dynamic processes in the debris disks of young stars.

A triple system invisible to the naked eye

Although appearing solitary, Fomalhaut is actually part of a very loose triple star system. Two stars accompany it at a great distance: TW Piscis Austrini (Fomalhaut B), an M4V red dwarf 0.28 light-years away, and LP 876-10 (Fomalhaut C), another red dwarf 0.77 light-years away. Their common motion through the Galaxy suggests that they form a gravitationally bound system, albeit very sparse. This system illustrates the diversity of stellar configurations and highlights the importance of astrometric studies like those of Gaia to detect non-visually obvious stellar associations.

A cultural and scientific landmark

Fomalhaut is mentioned in many ancient cultures, from Babylonian astronomers to Arab navigators. Its name comes from the Arabic "Fum al-Hūt," literally meaning "the mouth of the fish." It is one of the few stars named on medieval celestial globes. In modern astronomy, its protoplanetary system makes Fomalhaut a major target for future observation missions in the infrared and submillimeter, such as those of the James Webb space telescope. At the crossroads of stellar astrophysics, disk dynamics, and exoplanet research, Fomalhaut continues to fuel our understanding of the formation of planetary systems in our galactic neighborhood.