One of the exciting challenges 21st Century astronomers face is how to explain the sheer variety of planets and planetary systems. Some Nasa astronomers are trying to do that by observing young stars in the earliest stages of planetary formation. But they need your help. Become a Disk Detective and search for the origins of alien worlds.
Planets form from the rubble left behind by new-born stars. Particles of gas and dust circle the young star in a dense disk. Gravity from clumps of larger particles attracts smaller particles, adding to the clump’s mass and increasing its gravitational pull. Over time this process, called accretion, sweeps up enough mass to form asteroids, comets, and planets. We thought this process was simple and straightforward when we could only study one planetary system - our own. The Nice Model (“neece” like geese, not “nighce” like rice) described how terrestrial planets form close to a star while giant planets form farther out.
Reality is much different
Few of the thousands of planetary systems discovered so far come close to looking like our Solar System. The largest planetary system has 8 planets - some discovered by the Planet Hunters crowdsourced project - orbiting their parent star closer than Mercury orbits the Sun. Astronomers have discovered gas giants dozens of times bigger than Jupiter that skim the surface of their parent stars. The most common planets in the galaxy seem to be ice giants several times larger than Neptune.
A team led by scientists at Nasa’s Goddard Space Flight Center hope to explain this diversity by looking at the beginnings - the planetary disks surrounding young stars. They started with data collected by a space telescope called the Wide-field Survey Explorer. Wise scanned the entire sky at infrared wavelengths, amassing a catalog of over 745,000,000 objects. Scanning that much data manually would take forever. The scientists figured they could hand the task to a computer since crunching numbers is what computers are built for. There was just one problem.
The computer couldn’t get the job done.
Planetary disks look like faint, fuzzy blotches in telescope images. So do galaxies, star clusters, nebulae, and a host of other things. Automated computer algorithms can’t tell the difference between one faint fuzzy and another. The software flagged hundreds of thousands of potential planetary disks, but the scientists had to inspect each image visually to make sure it had a good candidate. They couldn’t review that many images.
Enter the crowdsourcers.
The Goddard scientists and Zooniverse created Disk Detective to let the public get the job done. After a brief introduction, you review infrared and optical images of potential planetary disks. The images come from 3 different astronomy archives: the Digitized Sky Survey (DSS2), the 2-Micron All-Sky Survey (2Mass), and the Wise telescope. None of the images show a planetary disc directly. Instead you evaluate the light from a target star. If it doesn’t move or split into multiple objects as you switch from one image to the next, then you mark it as a good candidate.
It sounds complicated, but none of this requires astrophysics expertise - just the innate human ability to recognize patterns. If a dozen people classify an object as a potential planetary disk, there’s a good chance that they are right. The power of crowdsourcing lets the project’s scientists combine millions of amateur best-guesses to produce high-quality results on a scale experts could never achieve.
Disk Detective launched in January 2014 with an initial dataset of 32,000 candidates. Within the project’s first two weeks, amateurs produced a quarter million classifications. The project scientists added more candidates in May, bringing the number to review over 300,000. Since each candidate has 5 images from the DSS2, 2Mass, and Wise archives, the amateur Detectives will review over 1,500,000 images, producing a catalog of thousands of planetary disks.
The amateur Disk Detectives will make new science possible.
The project’s scientists will do several things with the catalog the Disk Detectives produce. If the object is close enough and bright enough, they will apply for time on large ground based observatories and the Hubble Space Telescope (and eventually the James Webb Space Telescope) to take a picture of the disk directly. That may let them get pictures of young planets within the disk.
In the case of fainter, more distant disks, the follow-up observations will let the scientists explore conditions that change the shape of planetary disks. Finally, the Disk Detective catalog will let scientists apply statistical tools to analyze the thousands of planetary disks, mapping the disks properties against variables like stellar class or the age of the star.
Piece by piece, scientists will put together the puzzle and explain how planetary systems across the Milky Way formed - and better understand how our own Solar System came to be.