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Register a new userThe detectability of radio emission from exoplanets
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Like the magnetised planets in our Solar System, magnetised exoplanets should emit strongly at radio wavelengths. Radio emission directly traces the planetary magnetic fields and radio detections can place constraints on the physical parameters of these features. Large comparative studies of predicted radio emission characteristics for the known population of exoplanets help to identify what physical parameters could be key for producing bright, observable radio emission. Since the last comparative study, many thousands of exoplanets have been discovered. We report new estimates for the radio flux densities and maximum emission frequencies for the current population of known exoplanets orbiting pre-main sequence and main-sequence stars with spectral types F-M. The set of exoplanets predicted to produce observable radio emission are Hot Jupiters orbiting young stars. The youth of these system predicts strong stellar magnetic fields and/or dense winds, which are key for producing bright, observable radio emission. We use a new all-sky circular polarisation Murchison Widefield Array survey to place sensitive limits on 200 MHz emission from exoplanets, with $3sigma$ values ranging from 4.0 - 45.0 mJy. Using a targeted Giant Metre Wave Radio Telescope observing campaign, we also report a $3sigma$ upper limit of 4.5 mJy on the radio emission from V830 Tau b, the first Hot Jupiter to be discovered orbiting a pre-main sequence star. Our limit is the first to be reported for the low-frequency radio emission from this source.
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In this paper, we investigate the detectability of radio emission from exoplanets, especially hot Jupiters, which are magnified by gravitational microlensing. Because hot Jupiters have orbital periods much shorter than the characteristic timescale of microlensing, the magnification curve has a unique wavy feature depending on the orbital parameters. This feature is useful to identify radio emission from exoplanets and, in addition to magnification, makes it easier to detect exoplanets directly. We also estimate the expected event rate red of the detectable level of microlensed planetary radio emissions, assuming the LOFAR and the first phase of the Square Kilometre Array.
The majority of searches for radio emission from exoplanets have to date focused on short period planets, i.e., the so-called hot Jupiter type planets. However, these planets are likely to be tidally locked to their host stars and may not generate sufficiently strong magnetic fields to emit electron cyclotron maser emission at the low frequencies used in observations (typically >150 MHz). In comparison, the large mass-loss rates of evolved stars could enable exoplanets at larger orbital distances to emit detectable radio emission. Here, we first show that the large ionized mass-loss rates of certain evolved stars relative to the solar value could make them detectable with the Low Frequency Array (LOFAR) at 150 MHz ($lambda$ = 2 m), provided they have surface magnetic field strengths >50 G. We then report radio observations of three long period (>1 au) planets that orbit the evolved stars $beta$ Gem, $iota$ Dra, and $beta$ UMi using LOFAR at 150 MHz. We do not detect radio emission from any system but place tight 3$sigma$ upper limits of 0.98, 0.87, and 0.57 mJy on the flux density at 150 MHz for $beta$ Gem, $iota$ Dra, and $beta$ UMi, respectively. Despite our non-detections these stringent upper limits highlight the potential of LOFAR as a tool to search for exoplanetary radio emission at meter wavelengths.
Next-generation missions designed to detect biosignatures on exoplanets will also be capable of placing constraints on the presence of technosignatures (evidence for technological life) on these same worlds. Here, I estimate the detectability of nightside city lights on habitable, Earth-like, exoplanets around nearby stars using direct-imaging observations from the proposed LUVOIR and HabEx observatories. I use data from the Soumi National Polar-orbiting Partnership satellite to determine the surface flux from city lights at the top of Earths atmosphere, and the spectra of commercially available high-power lamps to model the spectral energy distribution of the city lights. I consider how the detectability scales with urbanization fraction: from Earths value of 0.05%, up to the limiting case of an ecumenopolis -- or planet-wide city. I then calculate the minimum detectable urbanization fraction using 300 hours of observing time for generic Earth-analogs around stars within 8 pc of the Sun, and for nearby known potentially habitable planets. Though Earth itself would not be detectable by LUVOIR or HabEx, planets around M-dwarfs close to the Sun would show detectable signals from city lights for urbanization levels of 0.4% to 3%, while city lights on planets around nearby Sun-like stars would be detectable at urbanization levels of $gtrsim10%$. The known planet Proxima b is a particularly compelling target for LUVOIR A observations, which would be able to detect city lights twelve times that of Earth in 300 hours, an urbanization level that is expected to occur on Earth around the mid-22nd-century. An ecumenopolis, or planet-wide city, would be detectable around roughly 50 nearby stars by both LUVOIR and HabEx, and a survey of these systems would place a $1,sigma$ upper limit of $lesssim2%$ on the frequency of ecumenopolis planets in the Solar neighborhood assuming no detections.
The detectability of planetesimal impacts on imaged exoplanets can be measured using Jupiter during the 1994 comet Shoemaker-Levy 9 events as a proxy. By integrating the whole planet flux with and without impact spots, the effect of the impacts at wavelengths from 2 - 4 microns is revealed. Jupiters reflected light spectrum in the near-infrared is dominated by its methane opacity including a deep band at 2.3 microns. After the impact, sunlight that would have normally been absorbed by the large amount of methane in Jupiters atmosphere was instead reflected by the cometary material from the impacts. As a result, at 2.3 microns, where the planet would normally have low reflectivity, it brightened substantially and stayed brighter for at least a month.
We present the results of a survey for low frequency radio emission from 17 known exoplanetary systems with the Murchison Widefield Array. This sample includes 13 systems that have not previously been targeted with radio observations. We detected no radio emission at 154 MHz, and put 3 sigma upper limits in the range 15.2-112.5 mJy on this emission. We also searched for circularly polarised emission and made no detections, obtaining 3 sigma upper limits in the range 3.4-49.9 mJy. These are comparable with the best low frequency radio limits in the existing literature and translate to luminosity limits of between 1.2 x 10^14 W and 1.4 x 10^17 W if the emission is assumed to be 100% circularly polarised. These are the first results from a larger program to systematically search for exoplanetary emission with the MWA.
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