Radio astronomy is difficult to do on earth, and it is getting harder every day. Our daily trust in radio technology makes radio interference a constant challenge, even in remote areas. And for some wavelengths, even the earth’s atmosphere is a problem, which absorbs or scatters radio light so that earth-based telescopes cannot observe those wavelengths well. To overcome these challenges, astronomers have suggested placing a radio telescope on the other side of the moon.
Proposed design of a radio telescope in a lunar crater. Photo credit: Saptarshi Bandyopadhyay
Since no atmosphere absorbs light, the moon provides a perfect view of the radio sky. The back of the moon is also shielded from all radio signals from Earth. If we were to build a radio dish in a lunar crater, similar to the way Arecibo was built in a natural valley, we would have the most sensitive radio telescope ever built. It would be so sensitive that we could use it to spot exoplanets.
Most of what we know about exoplanets comes from visible light, typically when an exoplanet passes in front of its star from our vantage point. Exoplanets can also be bright in the infrared, and so the exoplanets that we observed directly are seen in the infrared light. But there are times when an exoplanet can send out radio light. Jupiter, for example, is quite bright on the radio because of its strong magnetic field and intense polar lights. Some large exoplanets and brown dwarfs have strong aurora borealis that we can see from Earth.
Simulation of aurora on an exoplanet. Photo credit: Anthony Sciola
Earth-sized exoplanets are probably not bright enough in radio wavelengths to see from Earth, but would they be visible from a lunar radio telescope? The answer to this question was recently published in The Astrophysical Journal. In this study, the team simulated the effects of a planet’s magnetosphere on the radio light it emits, specifically the aurora produced during the planetary star’s active periods.
One of the things they found out was that auroras of visible light tend to cluster on the magnetic poles of a planet, but radio aurors occur further from the poles and are more dispersed. Radio aurora exist on Earth, but we cannot see them because the Earth’s ionosphere absorbs light. Our ionosphere would also block any radio aurora from Earth-like exoplanets. But a lunar observatory could see this radio aurora even if it comes from the dark side of the planet.
Based on the simulations of this study, the strength and stability of the magnetosphere could be determined from lunar radio observations. This is especially important for exoplanets in the potentially habitable zone of a star. Earth is a paradise for life because it has a rich atmosphere that is a direct result of its strong magnetosphere. By studying the magnetospheres of exoplanets, we could find out if they too have a rich atmosphere.
NASA is currently investigating how we could build a radio telescope on the moon. If it is built in the coming decades, we could finally see the northern lights of a distant earth.
Reference: Sciola, Anthony, et al. “Integration of the current-driven electron acceleration of the inner magnetosphere in numerical simulations of the radio emission from exoplanets.” The Astrophysical Journal 914.1 (2021): 60.
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