The K2-138 system: A near-resonant chain of five sub-Neptune planets discovered by citizen scientists by Christiansen, J, Crossfield, I, Barentsen, G, Lintott, C, Barclay, T, Simmons, B, Petigura, E, Schlieder, J, Dressing, C, Vanderburg, A, Allen, C, McMaster, A, Miller, G, Veldthuis, M, Allen, S, Wolfenbarger, Z, Cox, B, Zemiro, J, Howard, A, Livingston, J, Sinukoff, E, Catron, T, Grey, A, Kusch, J, Terentev, I, Vales, M, Kristiansen, M