We have
detected over 5,000 exoplanets in our galaxy, and possibly billions more await.
Whilst the first known exoplanets were gas giants, we now know that planets
about the same size as Earth (or equivalently, Venus) are fairly common.
Further, technological advances mean that we are starting to detect exoplanets
within the Habitable Zones of their host stars—the theoretical range of
distances from that star where water would be liquid on the planet surface
(under certain assumptions!). If we are searching for an Earth 2.0 with alien
life, then this may sound promising. However, planets are incredibly complex,
and even the sought-after combination of a Habitable Zone orbit with an
Earth-like mass or radius is certainly not sufficient to identify a planet as
truly “habitable”.
This video
focusses on one area of exoplanets which we are only just beginning to research
in the context of habitability: their mantles. The mantle of a small rocky
planet is the region between its metal core and its thin outer crust. (Here we
refer to “small planets” as those having a radius less than about twice that of
Earth’s—larger planets are likely gas- or ice-rich.) Many small planets should
have mantles of solid rock—in fact, we now expect the rock itself to be made of
broadly the same minerals as Earth’s mantle. Although these mantles are mostly
solid, they still convect like a liquid, yet over extremely long durations;
that is, over geological time-scales of tens to hundreds of millions of years.
Only where the temperature is hotter than the melting point of rock does magma
start to form.
Mantles
represent the largest part of a rocky planet by mass; we expect the mantle to
dominate in controlling the flow of matter and energy throughout the planet.
The convecting mantle, via its rising hot parts and sinking cold parts, will
transport material from the deep interior to near the surface, and back down
again. Mantle convection is therefore crucial in replenishing the water,
carbon, and minerals carrying life-essential elements at the planet surface,
out of the potentially-vast interior reservoir. In the video we will discuss in
more detail the example of planets’ deep carbon cycles, and their role in
regulating surface climate over geologic time.
