No Transit Timing Variations for Kepler-421b
Observations by NASA's Kepler space telescope have revealed the presence of several planets with orbital periods exceeding 500 days. Many of these planets with long orbital periods are near or beyond the snow-line of their host stars. The snow-line is basically the region in the protoplanetary disk around a star where temperatures start to become cool enough for volatiles such as water to condense into solid ice grains. Kepler-421b is a transiting snow-line planet with 4.2 times the radius of Earth and its orbital period around its host star is ~704 days. The planet's equilibrium temperature is estimated to be only ~185 K. Two transits of Kepler-421b were observed by Kepler.
Dalba & Muirhead (2016) present observations of the third transit of Kepler-421b by the 4.3-meter Discovery Channel Telescope on 19 February 2016. Observations of the third transit show no transit timing variations (TTVs). The lack of TTVs suggests Kepler-421b is either the only planet in its planetary system or the dynamical interactions with unseen planetary companions are too weak to noticeably perturb the orbit of Kepler-421b. Future transits of Kepler-421b will occur on 24 January 2018, 29 December 2019, 2 December 2021, and so on.
Dalba & Muirhead (2016), "No Timing Variations Observed in Third Transit of Snow-Line Exoplanet Kepler-421b", arXiv:1606.09246 [astro-ph.EP]