HD 191089 is a star in the constellation Capricornus, which may sound familiar to a lot of people because it is part of the zodiac. Since ancient Babylonian times [1], it has represented an animal that is half goat and half fish. Why anyone, even in ancient Babylon, would conceive of a mythological creature such as a goat-fish (or, as I like to think of it, as a mergoat), is beyond me.
Anyway, HD 191089 is a star that is very similar to the Sun but just a little more massive, a little hotter, and a little brighter [2]. It's located at a distance of 163 light years (50.1 pc) [3,4], and while that is too far away to use my Star Wars themed sound effects, it's still relatively close in astronomical terms. A lot of astronomers like to search for exoplanets around nearby Sun-like stars, and given that HD 191089 is a nearby Sun-like star, you might guess that astronomers found an exoplanet orbiting it, and you would be wrong. However, the star has the next best thing to an exoplanet: a debris disk.
Debris disks are disks of dust found within fully-formed planetary systems orbiting fully-formed stars [5]. To be clear, these debris disks should not be confused with protostellar disks (also called protoplanetary disks), which are disks of gas and dust where this material is collapsing inwards to form new stars with planets orbiting them. Also, debris disks mainly contain dust, while protostellar disks contain not only dust but relatively large amounts of gas. The dust in debris disks actually comes from collisions between objects in the planetary systems, typically things like comets or asteroids [5]. This dust will orbit for a short period of time (in astronomical terms) until various processes either remove or destroy the dust.
HD 191089 was originally identified as potentially having a debris disk in 1998 based on the fact that the star system produces excess amounts of infrared light [6], and infrared light in astronomy is emitted mainly by dust. However, the data used for this analysis came from the Infrared Astronomical Satellite, which produced very blurry images, so the star system and its debris disk only looked like a very bright infrared dot. In 2009, Laura Churcher, Mark Wyatt, and Rachel Smith used the Gemini South telescope to produce infrared images of HD 191089 demonstrating that the infrared light came from an extended area and indeed originated from a debris disk [7]. These observations were then followed up by Hubble Space Telescope and additional Gemini South observations that looked at light from the star that was passing through the dust disk and was reflected off of the individual dust grains in the disk in much the same way that beams of sunlight from a window passing through a very dusty room will reflect off of the dust grains falling within those beams of light [8].
From these observations, astronomers determined that the dust orbits HD 191089 in a flattened, disk-like ring with an inner diameter of 26 AU and an outer diameter of 78 AU [8]. The Earth orbits the Sun at a distance of 1 AU, while Neptune orbits at a distance of about 30 AU. Beyond Neptune is the Kuiper Belt, which is a collection of icy objects, including many comet-like objects as well as Pluto and a few other dwarf planets. The debris disk orbiting HD 191089 appears to be associated with Kuiper Belt-like objects in that star's planetary system. The dust is also cold, as you would expect for something that would be associated with comets; the average temperature is somewhere around -190 degrees Celsius [9]. Just for reference, this dust can still radiate lots of infrared emission even though it's way below room temperature.
As a bonus, the Hubble Space Telescope and Gemini South observations also detected a thin spherical halo of dust extending out to 640 AU from HD 191089 [8]. This halo of dust appears to be stuff that started off in the ring and then somehow got expelled from that ring.
So, astronomers may not have found an exoplanet in HD 191089, but they did find something nearly as good indicating that this star did indeed have some sort of planetary system. It does tell us something about how frequently we can expect to find planetary systems in orbit around other stars. What I find particularly fascinating about HD 191089 is not just that it has a debris disk but that it is associated with a group of other stars that also appear to have debris disks [9]. This group of stars is called the Beta Pictoris moving group, and it is named after the brightest star in the group, Beta Pictoris. The stars are called a moving group because they are all moving across the sky in the same direction at about the same speed. They all seemed to form together in a cluster about 23 million years ago and have since been spread out across the southern sky [9]. Astronomers have determined that 75% of the stars in the Beta Pictoris moving group have debris disks [9], and Beta Pictoris itself has a very famous debris disk that was the first one ever imaged by astronomers [10].
People are still trying to understand why so many stars in this particular group have debris disks. It could be that the stars' planetary systems are relatively young and contain lots of comets and other objects that are still colliding with each other, and as a result, the stars in the Beta Pictoris moving group are more likely to have debris disks than the average star in the sky [9]. Alternately, is is possible that the stars formed in an environment where gravitational interactions did not disturb the stars' planetary systems while they were forming, which allowed them to have the large number of comet-like objects needed to collide and produce the dust seen in these stars' debris disks [9]. In any case, HD 191089 will be an important part of understanding all of this.