Closely orbiting hot-Jupiter exoplanets are likely to be spiralling inwards towards their host star as a result of tidal interactions with the star. A new paper by Maciejewski et al reports a possible detection of this orbital-period decay in WASP-12b.

The authors have acquired 31 new transit light-curves over four years, and detect a trend under which the latest transits occur about a minute early compared to an unchanging ephemeris.

Transits of WASP-12b. O–C is the observed time compared to that calculated from an unchanging orbital period. The time (x-axis) is given in both a count of days (BJD) and a count of transits.

This is the most convincing claim yet of a changing orbital period in a hot Jupiter. Whether it shows the spiral infall, though, is less clear. As the authors explain, other tidal interactions between the star and the planet, such as that causing apsidal precession, could account for the effect. Further, in close binary stars there are known to be similar period changes on decade-long timescales that are not fully understood, but which might be caused by Solar-like magnetic cycles on the star.

One suggestion that this is not spiral infall comes from the deduced value of the tidal quality factor, Q, which the authors calculate as 2.5 x 10⁵. This is lower than other estimates of Q as nearer 10⁷.

The way to settle the issue will be to accumulate more data over a longer timespan until the case for spiral infall becomes overwhelming. It will thus be important to continue monitoring WASP-12b, and the other short-period hot Jupiters, over the coming decades.