Weather on Other Worlds. Ii. Survey Results: Spots Are Ubiquitous on L and T Dwarfs

We present results from the Weather on Other Worlds 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\\%-₂₀\\%^+17\\%, 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\\%-₁₇\\%^+25\\%) of our single T0-T8 dwarfs are variable with amplitudes between 0. 8% and 4. 6%. After correcting for sensitivity, we find that 80\\%-₂₇\\%^+20\\% of L dwarfs vary by ≥0. 2%, and 36\\%-₁₇\\%^+26\\% of T dwarfs vary by ≥0. 4%. Given viewing geometry considerations, we conclude that photospheric heterogeneities causing >0. 2% 3-5 μm 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.