https://www.space.com/asteroid-threat-simulation-nasa-deflection-idea.html
What makes a planet habitable?
The Milky Way Galaxy teems with planetary systems, most of which are unlike our own (1). It is tempting to assume that life can only originate on a planet that is similar to Earth, but different kinds of planets may be able to sustain Earth-like features that could be important for habitability. To focus the search for extraterrestrial life, scientists must assess which features of Earth are essential to the development and sustenance of life for billions of years and whether the formation of such planets is common. External effects such as stellar variability and orbital stability can affect habitability, but internal planetary processes that sustain a clement surface are essential to life; these processes are, however, difficult to characterize remotely. A combination of observations, experiments, and modeling are needed to understand the role of planetary interiors on habitability and guide the search for extraterrestrial life.
As exoplanet detection techniques improve, Earth-sized planets are likely to be found in the radiative habitable zone, that is, at distances from their host stars where they could have temperate (about 0° to 100°C) surface temperatures (2). This is important because to be habitable, a planet must be able to buffer life from extreme (globally sterilizing) variations in temperature. Launched in 2018, NASA's Transiting Exoplanet Survey Satellite has the capability to find small planets in the habitable zone of nearby stars and measure their radii (3). Ground-based telescopes are providing masses for such planets. Their densities provide a first-order constraint on composition, although it is likely that several different possible compositions can be inferred from the same density (4). Planets with compositions that differ from those of planets in our Solar System have been largely ignored, even though a wide range of stellar compositions and planetary densities have been discovered. The discovery of life elsewhere in the Solar System, for example on an icy satellite, would also radically expand the types of planets that need to be considered. The James Webb Space Telescope, due to launch in 2021, will attempt to detect atmospheres of the most favorable planets for life, but detailed measurements of atmospheric composition will require future extremely large telescopes on the ground and in space.
Atmospheric Signatures of Life
Atmospheric composition will be the primary observable that could imply the presence of life (5). However, identifying a biological signature in a planet's atmosphere requires an understanding of the possible compositions of abiotic atmospheres. The presence of free oxygen or an atmosphere out of chemical equilibrium could be signatures of life processes, but neither is definitive because atmospheres change over time and are open systems that are subject to complex sources and sinks. Volcanic eruptions release gases from the planetary interior that are the product of melting and magma migration. Atmospheric weathering can draw down noncondensable species, like carbon dioxide from Earth's atmosphere, to the seafloor, where they can be recycled back into the interior at subduction zones. All these processes are linked to the bulk composition of the planet and will evolve over time.
It remains unclear, therefore, what inferences can be made about the planet's habitability from its atmosphere before understanding more about how the atmosphere is tied to the interior dynamics and evolution of the solid planet. To advance this understanding, exoplanet atmospheres, which give a valuable snapshot of the surface composition, should be combined with experimental and modeling constraints on the interactions between the atmosphere and interior over long time scales.
Interior Processes Sustaining Life
On Earth, the environment needed for life to exist and be sustained is rooted in the presence of a stable hydrosphere and atmosphere, which are controlled by the planet's bulk composition, interior structure, and dynamics. Plate tectonics plays a crucial role in the long-term moderation of the climate by cycling material between the surface and interior, thereby helping to stabilize the long-term climate, and by cooling the deep interior as hot plumes well up to the surface and cold plates drop down to the core-mantle boundary (6). This cooling drives convection and dynamo action (conversion of mechanical to electrical energy) in the liquid-iron outer core, which in turn produces the geomagnetic field that shields the atmosphere and protects the surface from the solar wind (see the figure).
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https://science.sciencemag.org/content/364/6439/434?rss=1