Moving on up: looking for a new planet to inhabit

An investigation into the factors that must be considered before moving to another planet

By Oliver Ratcliffe

Brexit bumming you out? Climate change keeping you up at night? Fed up with Earth in general? Sure, it was fine when we were single-celled organisms getting our start, but after 4.8 billion years aren’t you craving a taste of the exotic? And if we do want to leave Earth, what do our options look like?

A useful resource for answering such a question is the Habitable Zone Gallery: a well-curated database of exoplanets (planets belonging to another solar systems than our own). Based on the work of astrophysicists Stephen Kane and Dawn Gelino, the database keeps track of the habitable zones of solar systems and is freely available at hzgallery.org. The habitable zone of a star refers to the optimal distance at which a planet orbits a star, where it is likely to be able to sustain liquid water. Depending on the planet’s route of orbit, it may stay in this zone for a variable proportion of its year. The planets most likely to harbour water are those that spend 100% of their orbit within their habitable zone. However, the habitable zone is not the only requirement for a planet to be considered friendly to life. It turns out that there are several other factors to consider.

One of the most important factors to consider is the temperature of the planet, both at its perihelion and at its aphelion, meaning the closest and furthest points from the star, respectively. For reference, Earth’s temperature is 288 Kelvin and only fluctuates by about 2.3 degrees throughout the year. Although humans have managed to carve out a sustainable life across the many climates of the Earth, the window of temperature in which we can survive is surprisingly slim; especially on the universal scale where temperatures vary from close to absolute zero all the way up to several thousands of degrees Kelvin.

Another is surface gravity. Here on Earth, gravitational strength, or g, is 9.81 metres per second squared. The planet Wasp-47b‘s surface gravity, for example, is estimated to be over twice this, at 21.23 metres per second squared. Experts suggest that humans could plausibly survive at a g up to four times that of Earth’s, but they emphasise that you would need above average muscle strength. You also don’t want the gravity to be too low, however, as surface gravity is essential for maintaining an atmosphere; without a strong enough pull, gas molecules will be able to escape the planet. An atmosphere is desirable as it allows for increased protection from cosmic radiation, as well as having a host of other benefits, such as allowing for liquid water.

Surface gravity is partially dependent on the planet’s mass but mass is also important on its own. There appears to be a ‘Goldilocks’ line to walk, in which the planet can’t be too large or too small. If the planet has too small a mass, it is more likely to be geologically dead i.e. it lacks the tectonic plate activity present here on Earth. These tectonic movements are crucial for the recycling of certain minerals and resources and they also contribute to the Earth’s magnetic field which helps protect us from high frequency radiation out of space. On the other hand, an overly large planet will likely produce a very powerful surface gravity and have a very oppressive atmosphere.

Finally, the planet’s rotation should not differ too much from that of Earth’s. Ideally, any new home would see an Earth-like moderate change of seasons which prevents extremes of temperatures occurring at any one point on the planet. The tilt leading to our 24-hour day-night cycle should also resemble Earth’s. Our circadian rhythms are finely tuned to such a cycle and are vulnerable to disruption – think of jet lag – but also as nights become longer, the temperature difference between day and night increases. If night lasts too long, any life on the dark side of the planet will struggle to survive.

Investigation into this topic demonstrates just how many necessities there are for any prospective planetary home, and without even considering actually travelling there. Hopefully, as it did me, it leaves you with an appreciation of just how uniquely suited Earth is to life and indeed how suited we are to Earth.

From Issue 18