Blog 19: Exomoons

You've heard of exoplanets, now get ready for exomoons! If the goal of our search for other planets is to possibly find other life, or just to have more knowledge, the next logical step would be to look at moons. Even within our solar system, we have 182+ moons and of those moons, more have signs of liquid water (e.g. Europa and Enceladus) than planets (e.g. Mars). With all the exoplanets we have been finding lately, exomoons could fructuous line of research, if we have the technology to explore it.

Jupiter and some of its moons. Source

So how do we find exomoons? Some projects are already underway. Here at the Center for Astrophysics, research is already being conducted to look for anomalies in stars' light curves that could point to exomoons in addition to exoplanets. Today, I am going to talk about this astrobite, which discuses future, direct detection of exomoons.

How would one directly detect an exomoon? They key to exomoon detection is the compositional differences between a moon and the planet it orbits. This would lead to distinct spectrographs for a planet and its moon. By analyzing the combined spectrum of the planet-moon system, there could arise evidence for moons. For example, the figure below shows the spectra for an Earth-Moon analog system orbiting Alpha Centuri. The bottom half shows the percent of the flux density due to the moon in this system.

By analyzing a system's flux at different frequencies, different elements of the system can stand out. For example, in the infrared wavelength in the system above, the moon accounts for 99.8% of the total flux in the water band at ~2.7 microns. Analyzing these differences can also give us the spacial separation of a planet and its moon. The offset of the origin of peak flux in different wavelengths can point to the location of a moon (as shown in the figure below).

The one problem with this exciting new research is that we don't quite have the technology to resolve these systems. Therefore, these plans are for future technologies. For example, to find the Earth-Moon analog orbiting Alpha Centauri, one would need a spacial resolution of ~2 milliarcseconds in the infrared. The Hubble Space Telescope, by comparison,  can only resolve ~100 milliarcseconds in similar wavelengths. The image below describes the size of telescope needed to resolve nearby candidates.

If we could achieve this technology, not only could we find these exomoons, but we could check for chemical compositions that could mean the moons are habitable or even containing biosignatures. The cliche saying to shoot for the moon could take on a new meaning in the near future.