Exploring Exoplanet Origin Stories

Citizen scientists are exploring exoplanets’ birthplaces, classifying more than 1 million infrared sources and finding 37 disk candidates (so far) for follow-up study.

protoplanetary disk around HL Tauri

Planets are forming around HL Tauri, a young, variable star 450 light-years away. The Atacama Large Millimeter/submillimeter Array (ALMA), which captured this incredibly detailed image, could follow up on disks discovered by citizen scientists participating in DiskDetective.org.
ALMA (NRAO/ESO/NAOJ) / C. Brogan / B. Saxton

Who would Superman be without his tragic past, or Spider-Man without the radioactive arachnid? Origin stories similarly define the character of the hot Jupiters, super-Earths, and ice giants that we see through the exoplanet-hunting telescopes of today and tomorrow.

But our understanding of exoplanet origins remains limited. Their birthplaces, the dusty disks that surround stars, are easy enough to find — astronomers just look for extra infrared emission from the dust. Roughly one in five sunlike stars has a detectable debris disk, and astronomers already know of a few thousand protoplanetary disks (those with dust and gas) and a few hundred debris disks (those with just dust). Only few of these are imaged at high angular resolution to suss out the details. Where disks form, around which stars, and when remain open questions.

Now the astronomers of DiskDetective, a citizen-science project led by Marc Kuchner (NASA Goddard) and the Zooniverse team, are taking a different, Kepler-like approach to exoplanet origins.

Taking a Census

Kepler’s goal was to build a huge sample of exoplanets. Individual exoplanets (particularly the Earth-size ones in Earth-like orbits that were its ultimate goal) would be too far away to study in any detail, but detail was never the real aim. The vast database is now yielding statistical answers to the question “how did we get here?”, revealing the fraction of stars that host planets and the fraction of those planets that might be habitable.

DiskDetective is applying the same statistical approach to the stellar disks that ultimately become planetary systems, amassing candidates to answer similar questions about how and when planets form.

DiskDetective screenshot

After a quick tutorial, you'll be ready to start classifying, differentiating circumstellar disks from galaxies, asteroids, nebulae, and other infrared sources.

DiskDetective uses data from the Wide-field Infrared Survey Explorer (WISE) satellite, which mapped the entire sky’s infrared emission and found hundreds of thousands of infrared sources that could be disks around nearby stars . . . or far-off galaxies, asteroids, interstellar dust clouds, and other extended infrared sources. To pick out the disks, DiskDetective employs citizen scientists (i.e., you or me, or anyone with an internet connection and some spare time) to classify each of 278,000 infrared sources in the WISE catalog.

Try it yourself: Go to DiskDetective.org and you’ll receive a snappy tutorial just a few minutes long on what circumstellar disks look like in WISE, Digitized Sky Survey, and 2MASS data. Then you’re ready to start classifying.

Many find citizen science projects like this one (and others at Zooniverse.org) a relaxing and fulfilling pastime. Kuchner says that, when the project released its first batch of 32,000 sources (20,000 of these in rotation at any one time), some volunteers complained they were seeing repeats. They weren’t wrong: within two months, those volunteers had already classified 20,000 objects each! Roughly 20 “super-users” have done about half the classifying, he says.

All of that work has netted more than 1 million classifications of WISE objects so far, turning up 478 “objects of interest” (possible disks) and 37 strong disk candidates. The strong candidates range in distance from 80 to 3,300 light-years. By 2018 the project is expected to net 1,000+ disks.

HR 8796A

The Gemini Planet Imager, one of the instruments that could conduct follow-up observations of strong disk candidates, took this image of the binary star HR 4796A and its dense, dusty disk.
M. Perrin / G. Duchene / M. Millar-Blanchaer / the GPI Team

As with Kepler, DiskDetective astronomers cannot rely on survey data alone. They are conducting follow-up observations on the candidates found so far to confirm that they really are disks rather than a background galaxy. They also take spectra to confirm the star’s spectral type.

Ultimately, the Hubble Space Telescope, James Clerk Maxwell Telescope, ALMA, and other telescopes may image promising candidates to reveal disk structure, including spiral shapes and disk gaps seen in other sources.

Learn more about citizen science opportunities in Sky & Telescope's March 2014 issue.

One thought on “Exploring Exoplanet Origin Stories

  1. Drew L.Drew L.

    Another excellent citizen scientist project that is currently underway that is worth mentioning is Planet Hunters. The Planet Hunters project was originally started four years ago by the Zooinverse citizen science program to enlist the public’s help in searching through the huge photometric database of NASA’s Kepler mission looking for transits caused by extrasolar planets. A couple of months ago, this project found a planet orbiting Kepler 289 they designated “PH3 c” which had been missed by the automated searches of the Kepler data set. With a mass of about 4 times that of Earth and a radius 2.7 times that of Earth, it is another example of a low-density world slightly larger than the Earth that is helping scientists define the mass-radius relationship of planets straddling the transition from rocky Earth-like planets and volatile-rich Neptune-like planets.


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