Rocky Exoplanets


This resource will introduce you to the detected and undetected rocky planets that populate the cosmos, and the cutting-edge tools we use in exoplanet research. We will discuss how understanding rocky planets requires collaboration between astronomers and geoscientists. We can only measure precious few properties of exoplanets using even the most advanced telescopes, yet the detailed observations of how processes on Earth operate (e.g., chemical reactions within rocks) can fill in some of the gaps. Meanwhile, the sheer sample size of exoplanets promises statistical leverage with which we can contextualise Earth’s place in the universe. After completing this resource, you might help us inch towards being able to answer: does the Pale Blue Dot represent one in a billion, one in ten thousand, one in a hundred?

Exoplanets and mantles

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.

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.

Video Resource

Resource activities

Exoplanet interiors

Discover more with this worksheet.


Activity questions

  • Why does combining different exoplanet detection methods give the most information about a planet?
  • Why could a rocky planet in the Habitable Zone still be inhospitable to life? (There are many reasons, give at least one)
  • What might happen to a planet if the chemical weathering of its crust shuts off (e.g., it becomes entirely covered by ice)?

Reflective questions

To answer and record these questions you will need to have an account and be logged in.

Task 1

What are the key arguments, concepts, points contained within it?

Task 2

What are you struggling to understand?

What could you do to improve your understanding of these concepts/terminology etc.?

Task 3

What further questions has this resource raised for you?

What else are you keen to discover about this topic and how could you go about learning more?

Can you make any links between this topic and your prior knowledge or school studies?

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Further reading

  • Climate app

    With this online tool, you can control an exoplanet’s greenhouse effect and incoming stellar radiation to explore how its climate is affected

  • Elements magazine

    An entire 2021 issue of the Geochemical Society’s Elements magazine, dedicated to the intersection of geoscience and exoplanet science. Contains several interesting and accessible articles which introduce the latest research in much more detail than could be contained here