Since the radio-frequency emission from planets is expected to be strongly inﬂuenced by their interaction with the magnetic ﬁeld and corona of the host star, the physics of this process can be eﬀectively constrained by making sensitive measurements of the planetary radio emission. Up to now, however, numerous searches for radio emission from extrasolar planets at radio wavelengths have only yielded negative results. Here we report deep radio observations of the nearby Neptune-mass extrasolar transiting planet HAT-P-11b at 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). On July 16, 2009, we detected a 3σ emission whose light curve is consistent with an eclipse when the planet passed behind the star. This emission is at a position 14′′ from the transiting exoplanet’s coordinates; thus, with a synthetized beam of FWHM∼16′′, the position uncertainty of this weak radio signal encompasses the location of HAT-P-11. We estimate a 5% false positive probability that the observed radio light curve mimics the planet’s eclipse light curve. If the faint signature is indeed a radio eclipse event associated with the planet, then its ﬂux would be 3.87 mJy±1.29 mJy at 150 MHz. However, our equally sensitive repeat observations of the system on November 17, 2010 did not detect a signiﬁcant signal in the radio light curve near the same position. This lack of conﬁrmation leaves us with the possibility of either a variable planetary emission, or a chance occurrence of a false positive.
Although we are not able to draw a deﬁnitive conclusion on 150 MHz radio emission from HAT-P-11 b, at the very least the hint of radio detection presented here identiﬁes HAT-P-11 b as a prime candidate for many follow-up observations in the near future. The priority is to try to conﬁrm the present tentative detection, via re-observation with GMRT at 150 MHz and/or new observations with LOFAR in the 30-250 MHz range (providing a 1-100 mJy sensitivity depending on the observation parameters used; van Haarlem et al., submitted, 2013) and UTR-2 in the 10-30 MHz range (providing a sensitivity of ∼100 mJy; Ryabov et al. 2004). The observed spectral range can also be extended toward shorter wavelengths using GMRT, with even higher sensitivities of ∼1 mJy at 240 MHz and ∼50 µJy at 614 MHz (e.g., Lecavelier des Etangs et al. 2009). Observations over a broad range of frequencies will, with any luck, allow conﬁrmation of the existence of the emission and better constraints on the planetary magnetic ﬁeld strength and determination of the radio spectral index.
The future observations should also be distributed at multiple epochs and at diﬀerent orbital phases of the planet to characterize the suspected variability of the radio emission in terms of duty cycle, as well as the radio emission as a function of the star-planet angle and the emission directivity (CMI produces narrowly beamed radio emission). In this context, Hess and Zarka (2011) have analyzed all the observables that could be derived from a broadband dynamic spectrum with a suﬃcient signal-tonoise ratio.
In the long term, it should be possible to investigate if the radio emission from extrasolar planetary systems correlates with the stellar spot activity. In this context, HAT-P-11 represents the target of choice because the Kepler mission has recently demonstrated the possibility of following the spot activity and perhap even of drawing the butterﬂy diagram of surface spots for this star.
Radio observations of diﬀerent types of exoplanets are also crucial for comparative exoplanetology. For instance, observations of the other transiting hot-Neptune GJ436 b (Gillon et al. 2007) will allow comparison with a planet similar in mass but orbiting an M-dwarf star which is half the mass and radius of HAT-P-11 and known to have a strong Far-UV and Lyman-αoutput (Ehrenreich et al. 2010). With a 6.55 Earth mass, the nearby exoplanet GJ1214 b should also provide an opportunity to undertake search for radio emission from a water-rich lower mass super-Earth planet (Charbonneau et al. 2009; D´esert et al. 2011). In the near future, more sensitive observations at even lower frequencies will become feasible with the LOFAR observatory, providing a great boost to exoplanetary research.