{"release":{"id":"DR1","released_on":"2026-08-21","doi":null,"url":"https://spacecatalog.org/releases#dr1"},"epoch":{"requested":"now","label":"J2026.674","julian_year":2026.674,"time":"2026-09-04T00:09:22.429Z","years_from_j2000":26.674,"frame":"date","applied":"Proper motion, then precession and nutation. The catalogued J2000 position is carried along the great circle the object's own published motion puts it on, at a constant velocity, and the result is referred to the true equator and equinox of this instant. Those are separate things: the motion moves the object, and the rotation turns the grid under it by about fifty arcseconds a year, with nutation adding up to another seventeen.","excluded":"Parallax, the radial-velocity perspective term, aberration and refraction; each is below the accuracy of the catalogued positions themselves. An object with no published proper motion comes back where the catalogue has it rather than at a fabricated zero, and says so with a null pm_ra_mas_yr. A body with no fixed position is not propagated at all — its place comes from an ephemeris, which is what /api/v1/visible answers with."},"slug":"k2-138-d","name":"K2-138 d","class":"neptune_like_planet","category":"exoplanet","object_type":"Neptune-like planet","mag":null,"mag_band":null,"abs_mag":null,"abs_mag_band":null,"distance_ly":660.7438977974647,"distance_method":"NASA Exoplanet Archive sy_dist (TICv8, 2019AJ....158..138S)","ra_deg":348.9490311,"dec_deg":-10.8497391,"epoch_ra_deg":349.298902,"epoch_dec_deg":-10.703357,"epoch_moved_arcsec":null,"pm_ra_mas_yr":null,"pm_dec_mas_yr":null,"orbits":"K2-138","semi_major_axis_km":8815802.520351,"periapsis_km":null,"mass_kg":4.729956951290772e+25,"mass_msun":0.0000237876,"constellation":"Aquarius","discovered":"2017","featured":false,"url":"https://spacecatalog.org/object/k2-138-d","radius_km":15243.659000000001,"radius_kind":null,"data_quality":null,"description":null,"properties":{"host_star":"K2-138","host_v_mag":12.246,"eccentricity":0.043,"uncertainties":{"mass_kg":[8.062426621518362e+24,8.301313336230016e+24],"radius_km":[535.7604000000001,663.3224],"distance_ly":[6.487250352786027,6.614451340095557],"eccentricity":[0.03,0.041],"host_mass_solar":[0.023,0.019],"inclination_deg":[0.6,0.6],"host_radius_solar":[0.018,0.031],"host_temperature_k":[13.1,41.8],"semi_major_axis_km":[74798.93535,59839.14828],"orbital_period_days":[0.00021,0.00021],"argument_of_periastron_deg":[38,82]},"host_mass_solar":0.935,"inclination_deg":89,"stars_in_system":1,"discovery_method":"Transit","host_radius_solar":0.863,"planets_in_system":6,"discovery_facility":"K2","host_spectral_type":"G8 V","host_temperature_k":5356.3,"orbital_period_days":5.40479,"transit_midpoint_bjd":2457743.15989,"equilibrium_temperature_k":888,"argument_of_periastron_deg":83},"designations":[{"catalog":"Exoplanet","designation":"K2-138 d"}],"source":{"id":"exoplanet_archive","name":"NASA Exoplanet Archive (Planetary Systems Composite Parameters)","url":"https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=PSCompPars","license":"Public domain (NASA)"},"updated_at":"2026-09-02T19:50:56.720Z","attribution":"Data from SpaceCatalog.org. Individual records carry their source and license in `source`; attribution is required for CC-BY and CC-BY-SA rows."}