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Astronomical observatories on the lunar surface and in cis-lunar space will be amongst the most sensitive probes for learning about the habitability on Earth-like exoplanets.

Earth-like Planets

Can we find another planet like Earth orbiting a nearby star?

Extrasolar planets (exoplanets) have moved from being a novelty to a fundamental part of modern astronomy. Over 1500 exoplanets have been discovered and confirmed—with several thousand additional candidates, largely from the Kepler mission. As the Astronomy Decadal Survey and National Space Policy emphasize, a major intellectual and societal thrust for the coming decades will be to discover and study Earth-like planets in the habitable zone—the region where water is liquid for long periods of time—around nearby, solar-type stars. Results from Kepler suggest that while the frequency of stars with exoplanets in general is high (~ 45%), the frequency of stars with terrestrial sized, habitable zone (HZ) exoplanets is very uncertain and may be much less. The search for Earth-analogs around solar-type stars will therefore require that we budget for this uncertainty, and observe hundreds of star systems in order to guarantee a statistically useful sample size. Further, surveying nearby stars to directly detect and characterize faint Earth-like planets requires routine observations on par with the Hubble Ultra Deep Field. The search for Earth-like worlds leads naturally to ambitious telescope projects, such as a farside low frequency array to characterize stellar weather and exoplanet magnetospheres for habitability and a large (perhaps ~12 m diameter or larger) ultraviolet/optical telescope possibly built in cis-lunar space. NESS will study the science background and technology development required to enable the remote detection and characterization of potentially habitable planets around other stars.

Characterization of Exoplanets

How can lunar radio arrays enable the detection and characterization of exoplanets and their magnetic environments?

It is becoming increasingly apparent from studies of young solar analogs, that the enhanced magnetic activity of the zero-age main sequence Sun, powered by its rapid rotation, was a major factor in defining the atmospheric properties of the terrestrial solar system planets. This was recently demonstrated in dramatic fashion by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, which confirmed that ion loss due to solar coronal mass ejections (CMEs) early in Mars history likely severely depleted its atmosphere. The presence of a stronger planetary magnetosphere may have prevented a similar process occurring on Earth. 

Extrapolating to the exoplanet population, the impact of stellar magnetic activity on planetary atmospheres, and the importance of planetary magnetic fields in negating such activity, may redefine habitability. For example, Kepler has shown that most M dwarfs harbor small planets, with 2.5 +/- 0.2 planets per M dwarf with radii 1-4 Earth radii and periods shorter than 200 days. Together, this implies that the nearest potentially habitable planet orbits an M dwarf, and indeed statistics from Kepler suggest such a system is < 3 pc from Earth. However, many M dwarfs are known to be particularly magnetically active, flaring frequently and with much higher energy than produced in solar flares. Moreover, M dwarfs have longer spin-down timescales (> 1 Gyr) and are thus magnetically active for a much longer period of time than G dwarfs like our Sun. Studies of possible flares and CME events on planets orbiting such stars suggest that these events severely impact the ability of such planets to retain their atmospheres, compounded by the fact that the habitable zone around M dwarfs is much closer to the parent star than the solar case (~0.2 AU). 

However, no CME on a star other than the Sun has ever been detected. Similarly, direct detection of planetary magnetic fields has yet to be achieved and remains the most crucial ingredient in assessing planetary habitability in the context of stellar activity. Both stellar CMEs and planetary magnetic fields can be probed via extremely bright radio emission at low radio frequencies. The Sun produces intensely bright radio bursts (Type II bursts) typically at frequencies below 100 MHz and associated with fast CMEs. These bursts are attributed to plasma radiation, an intensely bright coherent emission process whereby accelerated electrons cause radiation at the electron plasma frequency. Thus, the characteristic drift in frequency often observed in the dynamic spectrum of a burst reflects the large-scale transport of a body of plasma through density gradients in the solar corona and/or interplanetary medium. Only bursts detected at the lowest radio frequencies, inaccessible from the ground, can truly probe propagation through the interplanetary medium of nearby stars. 

Similarly, all the magnetized planets in our solar system, including Earth, have been found to produce bright coherent radio emission at low frequencies, predominantly originating at high magnetic latitudes and powered by auroral processes. The radio bursts, generally attributed to electron cyclotron maser emission, are found to be produced at the electron cyclotron frequency and thus enabled direct determination of magnetic field strength. As well as providing diagnostic information on the presence, strength and extent of planetary magnetospheres, the detected radio bursts are the only means to accurately determine the rotation period of the planetary interior for the gas giants. The detection of similar emissions from extrasolar planets has been one of the primary goals of a new generation of radio telescopes, such as the LWA, LOFAR and the SKA. These include the Owens Valley Radio Observatory-Long Wavelength Array (OVRO-LWA), led by Co-Investigator Hallinan, a telescope designed to image the entire observable sky every few seconds in the search for the radio bursts from nearby stars and orbiting exoplanets. However, it is notable that, of solar system planets, only Jupiter has the magnetic field strength to allow detection from the ground, with the radio emission from the other planets being produced below the ionospheric cut-off frequency (<10 MHz). Therefore, while these ground-based experiments may provide the first detection of gas giant exoplanet magnetospheres, the detection of a terrestrial planetary magnetic field will almost certainly require a lunar surface array. The requirements for observations at frequencies below those accessible from the ground suggest that much of the technology relevant for a Cosmic Dawn Lunar Telescope Radio Array is also applicable for studies of both stellar CMEs and the magnetospheric emission from extrasolar planets.