Habitable or Earth-Like?

A useful distinction, lost in the current breathless prose of the science media, is between Habitable and Earth-like planets. Back in the early 1960s Stephen Dole defined Habitable planets as having Earth-like atmospheres, gravities and at least 10% of their surface lying in a temperature-range suitable for human habitation. Since the 1980s, in the work of Martyn Fogg and his successors, there has been a broadening of the term to mean the less restrictive “able to sustain liquid water on its surface”. This has been the “official” meaning of “habitable” ever since Kasting, Whitmire and Reynolds, Habitable Zones Around Main Sequence Stars (1993) set the parameters of the modern debate. Thus a planet with a thick, unbreathable mostly CO2 atmosphere is habitable because it can keep liquid water on its surface, but it is emphatically not Earth-like.

However, truly Earth-like planets are still being studied – planets with Earth gravity and low-CO2 atmosphere. So what happens if the relative amount of water is varied? Desert planets, or for “Dune” fans, Arrakis-like planets, with low amounts of water, actually have wider habitable zones than water-planets, like Caladan. According to work by Yutaka Abe & Kevin Zahnle, Arrakis-like planets are able to retain liquid water – in the form of lakes near the poles – to much higher insolation levels than planets covered by water. Here’s their paper: Habitable Zone Limits for Dry Planets. Venus might have gone through a Dry Planet phase as recently as 1 Gya, before the greehouse runaway finally took hold. The critical limit is an insolation of 170% of what Earth currently experiences, implying desert surfaces above boiling point. The habitable zone will be clustered around the poles, depending on the axial tilt and season variations.

Another variable in planetary conditions is the eccentricity, e, of the orbit. The point of closest approach to a star, the periastron, is at a distance of a*(1-e) where a is the mean orbital distance. As insolation increases with the inverse square of the radius, the maximum insolation experienced is 1/(1-e)2 times the average. Contrariwise, the maximum distance (apoastron) is at a*(1+e) and thus the relative insolation drops by 1/(1+e)2. Thus the ratio of maximum to minimum insolation is ((1+e)/(1-e))2, which means an e~0.5 implies the periastron sunlight is 9 times stronger than apoastron sunlight. Yet according to the climate modelling work of Darren Williams, Earth-Like Worlds on Eccentric Orbits: Excursions Beyond the Habitable Zone, habitable regions can be found on planets with eccentricities of up to 0.7 – an insolation ratio over 32!

Another variable modeled in Williams’ work, and others since, is the effect of varying the axial tilt of a planet’s spin-axis, its obliquity. Earth’s present day tilt is 23.5 degrees, around which it oscillates on a multimillennial time-scale by a couple of degrees. In principle greater obliquities are perfectly possible – the planet Uranus has a tilt of more than 90 degrees, for example – with potentially dramatic climatic effects. While seasons on such worlds might be of an intensity we don’t experience on Earth, they remain essentially habitable worlds. The imagination boggles trying to imagine how life might adapt to scorching endless Summers followed by Sygian, glacial Winters, but Life finds a way. Combining eccentricity and obliquity at un-Earthly extremes makes for an exotic climate, famously used to best advantage by Hal Clement when he designed Mesklin, in “A Mission of Gravity”, a world covered in methane seas, except during periastron when the seas moved away from the liquid ammonia level of heat in the opposite hemisphere’s summer.

From deep-diving experience we know that humans, and other mammals (pigs are regular experimental subjects), can breathe mixtures of hydrogen/helium and oxygen. If we accept this rather exotic possibility then we should mention the Hydrogen Greenhouse planet, which Raymond Pierrehumbert and Eric Gaidos have studied. In their 2011 paper, Hydrogen Greenhouse Planets Beyond the Habitable Zone, they make the case for photosynthetic life being viable out to 10 AU or so thanks to a H/He rich atmosphere keeping the planet warm. With such broad habitable zones (from 2 AU to 10 AU in the case of the Sun) then such planets might prove more abundant than traditional “Earth-like” planets.

Thus, while Earth-like might seem a restrictive description, there could be quite a diversity of Earth-like worlds throughout the Galaxy.

One Reply to “Habitable or Earth-Like?”

  1. Hi, Adam.

    I just wish people would say what they mean. A surface liquid water planet is perfectly clear: it does what it says on the label. An Earth-analog planet is also pretty clear (if a little fuzzy around the edges, as with your desert planets example). A “habitable” planet is meaningless until you say habitable by what: do we mean one habitable by subsurface micro-organisms, or by surface-dwelling microbes, or by exotic forms of life unknown on Earth but satisfying any definition of “life”, or one which provides a shirt-sleeve environment for present-day humans?

    The problem with the terms “habitable planet” and “habitable zone” is that the writer tends to use them to mean “habitable by us without needing spacesuits”, at the same time implying that everywhere else is uninhabitable, by us or by anything else. The implied universe is the Star Trek cliche of worlds which are either Earth-like or Moon-like, with nothing inbetween. But we emphatically do not know this to be the case! This obsessive focus on finding planets which closely resemble Earth surely blinds the reader to all the other possibilities which may or may not be out there?

    Oxford, UK