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An introduction to exoplanets

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# 8.4  Habitable planets around M dwarfs

Because M dwarf stars are dim, a planet in the habitable zone needs to be close to its star. This means that the orbital period of a potentially habitable M dwarf planet is much shorter than an Earth year.

## Activity 8  The habitable zone of an M dwarf star

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The interactive application above is the same as the one you used in Activity 6, but the range of values on each slider has been changed to allow you to focus on the lowest-mass stars. M dwarfs have masses up to about half the Sun’s mass. As before, it includes a greenhouse effect in the calculation of the planet’s surface temperature.

Adjust the stellar mass slider to 0.5 solar masses, representing one of the most massive M-type stars. Adjust the distance of the planet from the star until the planet surface temperature is about 300 K. You can also use the arrow keys on the keyboard to amend the values.

How long is the planet’s orbital period?

You should obtain an orbital period close to 0.17 Earth years.

How long is the planet’s orbital period in Earth days?

Multiplying by the number of days in an Earth year, this is equivalent to about 62 Earth days.

Even for the most massive, most luminous M-type star, and assuming a greenhouse effect, a planet in the habitable zone would be so close to its star that it completes an orbit in about two Earth months.

Adjust the stellar mass slider to 0.12 solar masses, representing Proxima Centauri, the Sun’s closest neighbouring star. It is a low-mass M-type star. Adjust the distance of the planet from the star until the planet surface temperature is about 300 K.

How long is the planet’s orbital period?

For 303 K, the orbital period is 0.023 Earth years.

How long is the planet’s orbital period in Earth days?