A new study by a San Francisco State University astronomer has found that exomoons cannot exist in the recently discovered TRAPPIST-1 planetary system. So even though the hunt for exomoons is on (because whosoever discovers one will be the first to do so), calculations indicate that there is no reason to even bother looking for them in the closely packed exoplanets orbiting the red dwarf star TRAPPIST-1.
Stephen Kane, an associate professor in astronomy at the aforementioned San Francisco State University, took it upon himself to do the math regarding the possibility that exoplanets in the TRAPPIST-1 planetary system might have moons. According to Seeker, what Kane discovered was that warring gravities make it almost impossible for any moons to exist within the system -- and certainly no large moons like the Earth's own.
"I have several publications now on exomoons," Kane wrote Seeker in an email, "and for many years I've been thinking about how the ability of a planet to host a moon scales with the presence of nearby planets and proximity to the host star. The discovery of the TRAPPIST-1 system prompted me to finally calculate whether or not planets in compact planetary systems can actually harbor moons."
There is little doubt that exomoons exist, even though none have been detected to date. The existence of multiple planets around so many stellar bodies, if one takes the planets of the Solar System into account as indicative, points to the likelihood of there being quite a few satellites in orbit around faraway planets. In the case of TRAPPIST-1, though, its planetary train of tightly packed worlds -- all seven of the Earth-like exoplanets are circling in an area of space that would fit inside the orbit of Mercury -- present the problem of limited space.
Kane worked with two parameters, according to Seeker: "the Hill radius, or the area in space in which a planet exerts gravitational influence based on its mass and distance from the host star, and the Roche limit, which identifies where the gravitational effect near a planet is too strong for a moon to survive."
His calculations would not allow for bodies of substantial mass to orbit around any of the seven TRAPPIST-1 planets.
Kane explained, "A moon can only exist around a planet if it lies between these two boundaries: too close and it will be destroyed, too far away and it will escape the gravitational influence of the planet. The results of the study described in my paper show that, for most planets in compact planetary systems, the Hill radius and Roche limit are close enough to each other that there is no space in which a moon can exist and so such planets cannot have moons in orbit around them."
The TRAPPIST-1 planets excited the astronomical world in February when their existence was announced, not only because all seven planets were relatively Earth-sized and all in orbit in close proximity to their parent star, but it was also revealed that three, possibly even four, of the planets were in the star's habitable zone (where liquid water might develop, thereby providing conditions for the emergence of alien life). However, as Neil deGrasse Tyson has pointed out, the volatility of the TRAPPIST-1 star (it is relatively young) could have an atmosphere-stripping effect on the planets, rendering them irradiated and barren to the point of inhabitability.
Prof. Kane cautioned that the presence of the Moon as a companion to Earth was not necessarily a prerequisite for habitability and should not be used as a metric for exoplanets and their potential habitability. He acknowledged that although the Moon was beneficial and may have contributed to the emergence of life on Earth, it is as yet unknown just how important a role a moon or set of moons might play with regard to the existence of living organisms on distant planets.
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