1. Debris Belts Around Vega 0 Topic: Exoplanets Concepts: Infrared observations, debris disks, exoplanet detection, planetary systems Missions: Spitzer, Herschel Coordinated by the NASA Astrophysics Forum An Instructor’s Guide for using the slide sets is available at the ASP website https://www.astrosociety.org/e ducation/resources-for-the- higher-education-audience/ https://www.astrosociety.org/e ducation/resources-for-the- higher-education-audience/

2. Vega is wearing 2 belts! Infrared observations reveal the star Vega has a “warm” inner dust belt in addition to the previously-observed outer “cold” dust belt. The discovery can be explained by two belts of leftover planetesimals whose ongoing collisions produce the detected dust--analogous to the Sun’s asteroid and Kuiper Belts. Could this also suggest a similar planetary system for Vega? 1 Above: Image of the debris disk around the star Vega taken in infrared light with the Herschel Space Observatory. (Credit: Su et al., 2013) Right: Artist’s illustration of debris belt around Vega. (Credit: NASA/JPL-Caltech) Vega

3. How was the Discovery Made? The Spitzer Space Telescope and the Herschel Space Observatory detected infrared light in excess of that expected from the star alone. By comparing Vega to similar stars, scientists subtracted the infrared emission produced by the star itself, leaving the emission radiated by surrounding dust particles warmed by the star. The inner region of infrared excess could not be resolved into a belt like the outer one but the temperature of the dust is consistent with an asteroid belt about the same proportional distance distance from Vega as ours is from our smaller Sun. 2 Herschel Space Observatory infrared image of the debris disk around the star Vega, after subtracting the light of the star Vega itself. (Credit: Su et al., 2013) inner belt outer belt

4. The Big Picture Scientists have found other stars with similar inner and outer debris belts, with the outer belt about ten times farther from the stars than the inner belt. No planets have yet been detected around Vega, but if the star were eventually found to have several giant planets in orbit, this may suggest a common model for how stars form planets and how their planetary systems evolve. 3 One such star, HR8799, has been found to have four giant planets orbiting in the gap between belts. Our own solar system also has four giant planets orbiting between the rocky asteroid belt and the icy Kuiper belt.

5. How does this discovery change our view? Other stars can host planetary systems similar to the Sun. The outer belts of both Vega and Sun are about ten times more distant from the star than the inner belts. Our solar system has planets between debris belts, and Vega may have planets in the “gap” also. Multiple debris belts may be a clue to the presence of several planets. o Our solar system may not be unique. 4 In this diagram, the Vega system (left) is compared to our Solar system scaled up by a factor of four (right). The inner asteroid belt (orange) and Kuiper belt (red) are similar in relative scale, with the distance from the stars to the outer debris belts about ten times greater than the inner belts. The true relative size of our solar system is illustrated by the small drawing in the middle. Credit: NASA/JPL-Caltech

6. Resources 5 Press Release on this result: o http://www.nasa.gov/home/hqnews/2013/jan/H Q_13-006_Vega_Asteroid_Belt.html http://www.nasa.gov/home/hqnews/2013/jan/H Q_13-006_Vega_Asteroid_Belt.html Scientific Paper o Su, et al., 2013, ApJ 763:118 http://iopscience.iop.org/0004-637X/763/2/118/ http://iopscience.iop.org/0004-637X/763/2/118/ Synopsis article o http://astrobites.org/2013/01/20/asteroid-belt- found-in-the-vega-system/ http://astrobites.org/2013/01/20/asteroid-belt- found-in-the-vega-system/ Additional Resources o Exoplanet resource guide http://www.astrosociety.org/education/astrono my-resource-guides/the-search-for-planets- around-other-stars/ http://www.astrosociety.org/education/astrono my-resource-guides/the-search-for-planets- around-other-stars/ o AAS announcement by Kate Su https://www.youtube.com/watch?v=neJkNoFl2u M https://www.youtube.com/watch?v=neJkNoFl2u M o Extrasolar Planets Encyclopaedia http://exoplanet.eu/ http://exoplanet.eu/

7. Debris Belts Around Vega BONUS CONTENT 6

8. Why haven’t we found planets for Vega? Vega is oriented on the plane of the sky, so we’re looking at one of its poles, and its debris disk is seen face-on, not edge on. We can’t detect planets by the “transit” method since they wouldn’t pass in front of the star from our perspective. We can’t detect planets by the “Doppler” method because stars like Vega rotate very fast and have very few narrow lines in their spectra for noticing the tiny shifts in line position caused by the gravitational tugs of planets. 7 Image credit: NASA/JPL-Caltech Direct visual searches have turned up empty, suggesting orbiting planets are too small for us to yet detect by this method. The more powerful James Webb Space Telescope or more sensitive ground- based searches may eventually find Jupiter-sized or smaller planets orbiting in Vega’s gap.