Transit and secondary eclipse photometry in the near - infrared

Near-infrared photometry of transiting extrasolar planets can be of great scientific value. It is however not straightforward to reach the necessary millimagnitude precision. Here we report on our attempts to observe transits and secondary eclipses of several extrasolar planets at 2.2 μm. Best results have been obtained on OGLE-TR-113b using the SOFI near-infrared camera on ESO’s New Technology Telescope. Its K-band transit shows a remarkably flat bottom indicating low stellar limb darkening. Secondary eclipse photometry has resulted in a formal 3σ detection, but residual systematic effects make this detection rather uncertain. 1. Near infrared observations of transiting planets Near-infrared photometry of transiting extrasolar planets can be of great scientific value. First of all, since the emergence of wide field near-infrared cameras, transit surveys in J-band targeting M dwarfs have become feasible (see Hodgkin et al., this volume). The areas of their stellar disk are up to two orders of magnitude smaller than that of the sun, enabling detections of transits down to Earth-size planets. Furthermore, due to their low luminosities, the habitable zones around M dwarfs are significantly closer to the host star, increasing the transit probability for life-bearing planets. In addition, follow-up near-infrared transit photometry of known extrasolar planets is also valuable. The main difference from the optical is that the stellar limb darkening is significantly reduced, in particular at K-band. This results in a stronger contrast between the ingress/egress periods and the main part of the transit, allowing a better estimate of the transit impact parameter and the planet/star size ratio. This can also help to distinguish false interlopers among planet candidates in transit surveys. Figure 1 shows the model V and K band light curves of two systems, one of a mid M-dwarf transiting an A star at low impact, and the other of a Jupiter size planet transiting a G dwarf at high impact. While in the optical the two light curves are virtually identical, in K band they are easily distinguishable. The low impact transit of the M dwarf is significantly deeper in K due to the lower limb darkening, making the occulted surface area of the primary relatively brighter. Although it puts the strongest demands on photometric accuracy, arguably most interesting are observations of the secondary eclipse of transiting planets. Recent detections of the secondary eclipses of TrES-1b, HD209458b, HD149733b, and others using the Spitzer space telescope at mid-infrared wavelengths, constitute the first detections of direct thermal light from extrasolar planets (Char-