Small, cold planets for Fomalhaut

Fomalhaut is a bright star in the constellation the Southern Fish. It is best visible in October and November. It’s not very far from our own sun: just 25 light-years away. And like our own sun, Fomalhaut has planets!

Using ALMA, astronomers have discovered that the planets of Fomalhaut must be rather small. Maybe a few times larger than the Earth, but certainly not as large as Jupiter or Saturn. And since the planets are very far from their star, they must be cold, too.

ALMA has not seen the planets directly. They’re too small for that. But ALMA found that Fomalhaut is surrounded by a narrow ring of dust. The ring contains many dust particles as large as grains of sand. These particles emit millimeter and submillimeter waves, so ALMA can see them.

The astronomers found that the dust ring has very sharp edges. Somehow, the dust particles in the ring are prevented to spiral inward, in the direction of the star, or outward, away from the star. Only planets can do the trick.

One planet must orbit Fomalhaut close to the inner edge of the ring. The other planet orbits the star just beyond the outer edge. Together, they keep the ring in shape with their gravity.

If the planets were as small as Mars, they would not be able to confine the dust ring. But if they were as large as Jupiter, their gravity would destroy the ring. That’s how the astronomers know that the planets of Fomalhaut are probably a few times larger than the Earth.

The dust ring is very far from the star – some 20 billion kilometers. So the planets must also be very far away from the star. For comparison: the distance from the Earth to the sun is only 150 million kilometers (140 times smaller!). So the planets of Fomalhaut must be very cold.

In 2008, the dust ring of Fomalhaut has also been studied by the Hubble Space Telescope. Hubble may even have photographed the inner planet. But the sharp edges of the ring could only be seen by ALMA, even though the observatory was still under construction when the observations were made.


Fomalhaut is the brightest star in the constellation the Southern Fish. It is hotter, larger and more luminous than the sun. Fomalhaut is also much younger than the sun: less than 500 million years old. Its distance is 25 light-years.


This study was led by Aaron Boley. Aaron is an astronomer at the University of Florida in Gainesville. He worked with five colleagues from the United States and Chile. The results were published in a professional astronomy magazine, called Astrophysical Journal Letters.