'Earth-Like' Planets May Be Anything But, Astronomer Argues

Quit Calling Them 'Earth-Like' Planets

By Caleb A. Scharf

This year has been a spectacular one for exoplanets. New discoveries and new insights have truly pushed the gateway to other worlds even further open.

In the past 12 months we’ve gained increasingly good statistics on the incredible abundance of planets around other stars and their multiplicity. We also finally seem to have evidence that our neighboring star Alpha Centauri B does indeed harbor at least one world. It is by any set of standards, a great haul.

But I continue to be a bugged by the claims of ‘habitable’ worlds and ‘Earth-like planets’ that seem to beset many scientific announcements (including I’m ashamed to say my own). In the spirit of closing out the passage of our 4,500,000,000 th or so orbit around the Sun I thought I’d try to set the record straight, because I think we have so much more to look forward to than simply finding ‘another Earth’.

First, when press releases state that a ‘habitable’ world may have been found, the truth is far more complex. Astronomers and astrobiologists tend to use the term habitable as a shorthand for the presence of liquid water on a planetary surface, implying a range of temperatures between the freezing and boiling point of water. But this also requires a surface atmospheric pressure high enough for water to exist as a liquid without boiling off to vapor, and an atmosphere will alter the transfer of radiation to and from the surface – often by way of a greenhouse effect.

And this is just the beginning. By this simple criterion even the Earth is only partially habitable – about 85% of its area remains amenable to liquid water over a year (a fact that my colleagues Dave Spiegel, Kristen Menou and I reiterated a few years ago). So strictly speaking ‘habitable’ includes a range of environments that we would find appallingly hostile, including high-pressure, high-temperature climates and those in a sub-arctic category with thin atmospheres.

Sorry, but you're total conjecture (Credit: ESO)

The problem with the newly proclaimed habitable, or even potentially habitable exoplanets of 2012, is that not only do we at present have absolutely no information on the presence or absence of water or an atmosphere, we also have absolutely no idea (beyond informed guesses) about their geophysical history or present state. Geophysics is the dirty little secret here. On Earth the long-term (read millions of years) stability of the Earth’s surface environment close to the ‘habitable’ state is a direct consequence of geophysical re-cycling, the so-called Carbon-Silicate Cycle. Or to put this more crudely, no volcanism or tectonics, and you get no temperate climate.

This does not mean that some of them don’t hit the sweet spot, but it’s horrendously premature to say so.

Which gets us to the other point, the cavalier use of the phrase ‘Earth-like’. Utterance of this can evoke all sorts of images. It may make us think of oceans, beaches, mountains, deserts, forests, fluffy clouds, fluffy bunnies, warm summers, snowy winters, the local pub, or the fabulous hubbub of the local souk.

But this is typically far from the meaning attached by scientists. It can simply indicate a planet with a rocky surface, rather than a world with a thick gaseous envelope. It can mean a world that is roughly the same mass and density as Earth. It can mean a planet orbiting a star like the Sun. Or it can just mean that we got bored of saying things like ‘a two-Earth mass object in a close to circular orbit around a roughly 4 billion year old main-sequence star that is similar in mass to the Sun.’

Although at some level this is purely to do with semantics I think it’s important to consider. What I believe we really mean when we say ‘Earth-like’ is that a planet is Earth equivalent. That is to say that while the planet might feel completely alien to human senses it nonetheless matches many of the same physical and chemical characteristics of Earth. It’s a bit like renting a car at an airport where you’ve reserved the open top red sports-car, only to be told that they’ve run out but you can have ‘an equivalent’ vehicle. It’ll have four wheels, an engine, and yes you can wind the windows all the way down if you’d like.

And this is critical because one thing we have learned about exoplanets in 2012 is that they are remarkably diverse, and that the configuration and contents of our solar system are somewhat (only somewhat) unusual. For example, none of our planetary sisters belong to what may be the most numerous category of worlds – objects with masses between that of the Earth and that of Neptune. There’s an absolute load of those out there, but none here.

Our system is also relatively spread out, it is increasingly apparent that the universe likes to build compact orbital architectures of numerous planets with orbital periods of days to weeks. It is also apparent that a majority of planetary systems have likely gone through a far more dramatic period of dynamical rearrangement or dynamical ‘cooling’ than ours has – leaving a tell-tale signature in the highly elliptical orbits of major planets.

This is just the tip of the iceberg. Although we don’t yet know much about the elemental and chemical composition of exoplanets, we do see a diversity in the contents and conditions of proto-planetary environments. We also suspect that the pathway to any individual world includes an enormous variety of essentially random events – the process of building a planet is highly stochastic.

On the face of it this might seem a little depressing. After all, finding an ‘Earth-like’ planet is often described as the holy grail of exoplanetary science, the location of another place that could harbor life in the cosmos. But if all worlds are unique it might be that there are no other places quite like Earth, and if a delicate set of conditions are critical for life this would put a damper on these prospects.

Meet the parents. Carbon chemistry abounds. (Credit:NASA/HST)

However, the chemistry of the universe does seem to be the very same chemistry from which we’ve sprung. Molecules made with carbon are ubiquitous, whether in the sparse interstellar medium or the thickening clouds of forming proto-stellar systems, organic chemistry dominates.

It also permeates the ancient material of our solar system that we see preserved in carbonaceous chondrite meteorites and cometary contents. And life on Earth not only got going fast some 3.5 to 4 billion years ago, it seems to have rapidly evolved into an interlinked world-wide chemical network that robustly preserves the vital genetic blueprints for metabolism and survival – the core planetary gene set.

While it is true that we do not yet know the actual answer, my personal take on all of this is the following. There may be no ‘Earth-like’ planet out there, but there are almost certainly Earth-equivalent worlds – alien but nonetheless amenable to life.

Only the most contrived and strained interpretation of what we see by way of the elemental and chemical composition of the cosmos and of terrestrial biochemistry would suggest there’s something ‘special’ about what happened here on Earth. Carbon-chemistry rules, and Earth 4 billion years ago was about as alien from Earth today as one can imagine, yet life started up fast. It’s going to happen in other places too.

So, we should not necessarily hold our breath for other Earth-like planets, but we should expect an astonishing diversity of equivalent places. I for one can’t wait to find out how the complex rough and tumble of molecular evolution played out on those other worlds.

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