by Maddie Meagher (a 2016 Adler Astro-Journalist)
Trying to find planets forming around stars can be quite a daunting task even with the best technology available. This is where a Zooniverse citizen science project called Disk Detective comes in. It’s main goal is to find planets around other stars as well as finding planets in the process of forming. Currently we know very little about how exactly planet formation takes place. What we do know is that planets form around their parent stars in rotating vast gigantic disks, made of various gases and large chunks of rock. What we’re looking for in Disk Detective are two of kinds disks, and both types of these disks are the signposts of the planet forming process.
The data for Disk Detective comes from a survey from a NASA satellite mission called WISE (Wide-Field Infrared Survey Explorer). From 2010 to 2011 WISE created maps of the night sky in infrared wavelengths to look for theses disks. As stars with disks around them shine brightly infrared light due the dust in the disks, where a star all by it’s lonesome wouldn’t shine bright in infrared at all. The two major disks astronomers were looking for in these maps are YSO and Debris disks. YSOs have disks mostly made of gas where planets like Jupiter and Saturn can form. These disks are often less than 5 million years old and will tend to form in clusters. In the picture above for the YSO disk (HL Tauri) you can see where planets in development have begun to clear their orbits around their parent star. Seen by the empty bands in the disk.
On the other hand, Debris disks tend to resemble the Kuiper belt in our own solar system, however, on a more massive scale. A Debris disk’s age tends to be about 5 million years and older. They tend to be composed of more rocky and icy materials and they usually orbit around older stars. Rocky planets like Earth are believed to form out these disks by dust moats gathering around a star to form rocks. Collisions of the larger rocky objects eventually snowball in a rocky planet like Earth, Venus, or Mars!
Image: Distribution of infrared brightnesses (x-axis) for a sample of 84 stars. While most stars cluster around 1, a lot of the stars have a higher amount of infrared emission (further to the right on the x-axis). Stars with planets are shaded as dark gray, while stars without known planets are shaded with light gray. Although stars with known planets make up less than a third of the sample, four of the five stars with the highest infrared brightness have known planets.
Citation: Beichman et al.2005 Astrophysical journal 622: 1160-1170
Keep in mind, in the graph above stars with disks like HL Tauri are most likely not counted in stars with known planet category. As the bands you saw in the picture above are only indirect evidence of planets residing there.
While WISE has taken Thousands of pictures of the night sky to create the most powerful survey for dusty disks ever known, it does not change the fact stars surrounded by a disk can be rather difficult to spot. Stars with a disk aren’t the only objects in the night sky that glow bright in the infrared (ex: galaxies,asteroids,active galaxy nuclei). Even computer algorithms designed to automatically search for these disks are easily thrown off by these sources of confusion. This is why there is a need for citizen scientists to help classify these objects. So that we can make sure that what we’re really looking at stars with disks. Finding these disks and the birthplaces of planets has been a major quest of astronomers for the last three decades. So start classifying and make new discoveries!
Disk Detective Tutorial
Spectral Energy Distributions (SEDs)