Detecting the Presence of an Unseen Hot-Jupiter

Hot-Jupiters are a class of Jupiter-like planets that orbit very close to their host stars. They have orbital periods of only a few days. The most commonly accepted mechanism regarding the formation of hot-Jupiters is that they formed at larger distances from their host stars before migrating inwards. Alternatively, hot-Jupiters may also form in situ via gas accretion onto massive cores with 10 to 20 times the mass of Earth. Hot-Jupiters that formed in situ are expected to be accompanied by low-mass companion planets with orbital periods of less than ~100 days.

Millholland et al. (2016) present the possible detection of a non-transiting hot-Jupiter in a planetary system consisting of two low-mass transiting planet candidates with longer orbital periods. The technique employed to detect this non-transiting hot-Jupiter is a novel one which combines optical phase curve analysis and astrometric transit timing variations (TTVs). Optical phase curve analysis involves measuring the reflected light from the hot-Jupiter as its dayside rotates in and out of view.

Astrometric TTVs occurs when the transit timings of the outer planets are not perfectly periodic as the star they orbit is a "moving target". This is because the mass of the hot-Jupiter is sufficiently large to be non-negligible in comparison to the mass of the host star. As a result, the gravitational influence of the hot-Jupiter causes the star to wobble as both the star and hot-Jupiter are orbiting their common center-of-mass.

The non-transiting hot-Jupiter in this study was detected around a Sun-like star identified as KOI-1858. Orbiting KOI-1858 are two known transiting planet candidates - KOI-1858.01 and KOI-1858.02. KOI-1858.01 has ~3.53 times the radius of Earth and has a 116.3 day orbital period. KOI-1858.02 has ~2.06 times the radius of Earth and has a 86.0 day orbital period. Both the observed optical phase curve and astrometric TTVs are mutually consistent with the presence of a non-transiting hot-Jupiter with 1.5 ± 0.4 times the mass of Jupiter and a 2.991 day orbital period.

Reference:
Millholland et al. (2016), "On the Detection of Non-Transiting Hot Jupiters in Multiple-Planet Systems", arXiv:1602.05674 [astro-ph.EP]