Rotating Stars Can Help Planets Become Habitable

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What characteristics must a terrestrial planet exhibit to have the potential to host life? Orbiting within the habitable zone of its host star is certainly a good start, but there’s another important aspect: the planet has to have the right atmosphere. A recent study has determined how host stars can help their planets to lose initial, enormous gaseous envelopes and become more Earth-like.

Collecting An Envelope

When a terrestrial planet forms inside a gaseous protoplanetary disk, it can accumulate a significant envelope of hydrogen gas — causing the planet to bear more similarity to a mini-Neptune than to Earth. Before the planet can become habitable, it must shed this enormous, primordial hydrogen envelope, so that an appropriate secondary atmosphere can form.

So what determines whether a planet can get rid of its protoatmosphere? The dominant process for shedding a hydrogen atmosphere is thermal mass loss: as the planet’s upper atmosphere is heated by X-ray and extreme-ultraviolet (XUV) radiation from the host star, the envelope evaporates.

A Critical Dependence

In a recent study led by Colin Johnstone (University of Vienna), a team of scientists has developed models of this evaporation process for hydrogen planetary atmospheres. In particular, Johnstone and collaborators examine how the host star’s initial rotation rate — which strongly impacts the star’s level of XUV activity — affects the degree to which the planet’s hydrogen atmosphere is evaporated, and the rate at which the evaporation occurs.

The authors’ findings can be illustrated with the example of an Earth-mass planet located in the habitable zone of a solar-mass star. In this case, the authors find four interesting regimes (shown in the plot to the right):

  • Atmosphere evolution

    Evolution of the hydrogen protoatmosphere of an Earth-mass planet in the habitable zone of a solar-mass star. The four lettered cases describe different initial atmospheric masses. The three curves for each case describe the stellar rotation rate: slow (red), average (green), or fast (blue). [Johnstone et al. 2015]

    Case A
    (I
    nitial atmospheric mass of 10-4 Earth masses)
    Entire atmosphere evaporates quickly, regardless of the rotation speed of the host star.
  • Case B
    (I
    nitial atmospheric mass of 10-3 Earth masses)
    Entire atmosphere evaporates, but the timescale is much shorter if the stellar host is fast-rotating as opposed to slow-rotating.
  • Case C
    (Initial atmospheric mass of 10-2 Earth masses)

    If the stellar host is fast-rotating, entire atmosphere evaporates on a short timescale. If the host is slow-rotating, very little of the atmosphere evaporates.
  • Case D
    (Initial atmospheric mass of 10-1 Earth masses)

    Very little of the atmosphere evaporates, regardless of the rotation speed of the host star.

These results demonstrate that the initial rotation rate of a host star not only determines whether a planet will lose its protoatmosphere, but also how long this process will take. Thus, the evolution of host stars’ rotation rates is an important component in our understanding of how planets might evolve to become habitable.

Citation

C. P. Johnstone et al 2015 ApJ 815 L12. doi:10.1088/2041-8205/815/1/L12

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