Cool stars are smaller, dimmer, and redder than the Sun and many frequently emit superflares, events 10-1000X the energies of the largest solar flares. Cool stars are the most common type of star, hosting most of the universe’s Earth-size planets. Stellar flares are thought to create a “flare habitable zone” for these worlds not unlike the traditional temperature-based habitable zone where planets are too warm or too cold to maintain surface life. Outside the flare habitable zone, too much flare radiation may destroy Earth-like planetary atmospheres. Too little flare radiation results in insufficient UV light to power pre-biotic chemistry and spark life in the first place.
Howard was awarded a grant (GO 3174) through the Cycle 3 NASA TESS Guest Investigator Program to determine if the temperatures of superflares might alter the amount of deadly UV radiation reaching potentially-habitable planets orbiting cool stars. Superflares are commonly assumed to emit radiation at a temperature of 9000 Kelvin, but this number is very uncertain; some superflares have been known to reach a scorching 50,000 Kelvin. Very hot flares will bathe orbiting planets with ten times more UV radiation than would have been expected from a 9000 Kelvin flare. The danger posed to surface life by superflares will be drastically under-estimated if the temperature is much higher than expected.
Howard proposed observations of several hundred flare stars be made at the same time by NASA’s Transiting Exoplanet Survey Satellite (TESS) and UNC’s Evryscope facility in Chile. Evryscope is an all-sky array of small telescopes that simultaneously images the entire night sky every two minutes. Because Evryscope and TESS will observe many superflares at the same time but in different colors, they will be able to determine each flare’s temperature from its color. Just like a hot oven glows white while a cool oven glows orange, the hotter flares will be bluer. Once the temperature of the flare is known, the amount of energy released at biologically-relevant UV wavelengths may be estimated.
For more information regarding the approved NASA TESS Guest Investigator Program Grant Awards please visit https://heasarc.gsfc.nasa.gov/docs/tess/approved-programs.html