WEATHER ON OTHER WORLDS. II. SURVEY RESULTS: SPOTS ARE UBIQUITOUS ON L AND T DWARFS

We present results from theWeather on OtherWorlds Spitzer Exploration Science program to investigate photometric variability in L and T dwarfs, usually attributed to patchy clouds. We surveyed 44 L3- T8 dwarfs, spanning a range of J - Ks colors and surface gravities. We find that 14/ 23 ( 61%+ 17% - 20%, 95% confidence) of our single L3- L9.5 dwarfs are variable with peak- to- peak amplitudes between 0.2% and 1.5%, and 5/ 16 ( 31%+ 25% - 17%) of our single T0- T8 dwarfs are variable with amplitudes between 0.8% and 4.6%. After correcting for sensitivity, we find that 80%+ 20% - 27% of L dwarfs vary by 0.2%, and 36%+ 26% - 17% of T dwarfs vary by 0.4%. Given viewing geometry considerations, we conclude that photospheric heterogeneities causing > 0.2% 3- 5 m flux variations are present on virtually all L dwarfs, and probably on most T dwarfs. A third of L dwarf variables show irregular light curves, indicating that L dwarfs may have multiple spots that evolve over a single rotation. Also, approximately a third of the periodicities are on timescales > 10 hr, suggesting that slowly rotating brown dwarfs may be common. We observe an increase in the maximum amplitudes over the entire spectral type range, revealing a potential for greater temperature contrasts in T dwarfs than in L dwarfs. We find a tentative association ( 92% confidence) between low surface gravity and high- amplitude variability among L3- L5.5 dwarfs. Although we can not confirm whether lower gravity is also correlated with a higher incidence of variables, the result is promising for the characterization of directly imaged young extrasolar planets through variability.