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By Jove! Can climate change lead us to life on other planets?

Thanks to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, we’ve recently heard a great deal about how the Earth’s climate is changing. The IPCC’s cautious assessment of the…

Not just a pretty planet – Jupiter may provide clues for detecting Earth-like planets. NASA

Thanks to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, we’ve recently heard a great deal about how the Earth’s climate is changing. The IPCC’s cautious assessment of the situation is that we now know, with 95% certainty, that human greenhouse gas emissions are causing global warming - but did you know that the actions of other bodies in our solar system also have huge effects on our climate?

To be fair, this type of warming and cooling takes place over tens or hundreds of thousands of years, far longer than a human lifetime. But, although effects on this scale can feel irrelevant to us right now, we may well be able to use this information to help us in our future search for life on other planets.

A casual glance at the ever-growing catalogues of planets known orbiting other stars reveals that both the number and variety of those planets are increasing at a remarkable rate.

And while we don’t currently know of any planets that could be considered truly Earth-like beyond our own solar system, it is only a matter of time before such planets are discovered, and the search for life beyond our solar system begins in earnest.

Rapid rate of discovery

Throughout the history of astronomy, it is almost always the case that, though the discovery of the first of a given class of object is incredibly challenging, many more of that same class will follow quickly on the heels of the first.

Ceres, the largest object in the asteroid belt. NASA

Take, for example, the asteroids. The first asteroid, 1 Ceres, was discovered by Italian Catholic priest Guiseppe Piazzi in 1801.

In the few years that followed, three more asteroids were found – 2 Pallas, 3 Juno and 4 Vesta. The fifth asteroid, 5 Astraea, was not discovered until 1845 – but since that discovery, the population of known asteroids has grown rapidly, such that more than 600,000 have been found to date.

More relevant to our current story, the first planet discovered orbiting a Sun-like star, 51 Pegasi b, was found just 18 years ago – yet we already know of more than 700 (or more than 900, depending on which catalogue you use) such planets.

Finding Earth-like worlds

The search for life on these distant worlds will be an incredibly challenging process. In the past decade, several measurements have been made of the atmospheres of planets orbiting other stars.

Those observations have generally targeted “hot Jupiters” – planets far larger and more massive than Earth, orbiting far closer to their host stars. Both these factors make the kind of observations carried out easier than they would be for an Earth-like planet.

But how will we decide which planets represent the best prospects for the detection of life beyond our solar system? While it is possible to imagine an enormous variety of life occupying ecological niches vastly different to those found on Earth (something well serviced by science fiction), the one place that we know life exists and thrives is our own planet.

x-ray delta one

It therefore makes sense to direct our search to the most Earth-like planets (exo-Earths) we can find since those will offer the best odds of us making a positive detection.

Milanković cycles

We need to build a “checklist of habitability” by which we can assess the many exo-Earths we will detect, and rank them such that the most promising can be identified and targeted by the exhaustive observations needed in the search for life.

Obviously, practical concerns will play a significant role in the target selection – the closer the planet to our solar system, the easier it will be to observe. However, there are many other criteria that will have to be taken into account for us to determine the best place to look.

With that in mind, we have recently started studying the influence of the giant planets in our solar system on the Earth’s orbit, and therefore on our planet’s climate. It is well known that, over the past few million years, the Earth has experienced a series of glaciations (intervals of time within Ice Ages marked by colder temperatures and glacier advances), with short inter-glacial periods in which the ice retreated, followed by lengthy spells when the ice caps grew outward to bury the north of Europe, America and Asia.

Wikimedia Commons

The cause of those glaciations has long been argued to be the Milanković cycles – the periodic changes in the Earth’s orbit that are driven by our planet’s interaction with the other objects in the solar system.

Over time, our orbit twists and flexes under the perturbation of the other planets, and the Earth’s rotation axis nods back and forth. This, in turn, causes the average amount of energy the Earth receives from the Sun over the course of a year to vary on timescales of tens of thousands of years. It is that periodic variation that is thought to drive the growth and retreat of the ice caps causing the ongoing series of glaciations.

Wikimedia Commons

The strength and frequency of the Milanković cycles is tuned by the influence of the other planets in our solar system, and it is easy to imagine scenarios in which those planets moved on slightly different orbits, resulting in the Earth experiencing far greater orbital excursions.

Given that the exo-Earths we will discover around other stars will move in planetary systems vastly different to our own, it is therefore interesting to consider how the Milanković cycles would differ had our own planets ended up on different orbits.

Jupiter’s effect

Dave Waltham (of Royal Hollway, University of London) and I have begun a series of studies that will examine how the architecture of our solar system would affect the Milanković cycles at the Earth.

In our preliminary study, the results of which were presented for the first time at this week’s Australian Space Science Conference, we have looked at what would happen to the Milanković cycles were Jupiter located further from or closer to the Sun – but the rest of the solar system remained as we see it today.

We set up almost 40,000 scenarios, each of which featured the planets of our solar system initially moving on their current orbits, but with Jupiter shifted from the orbit we see today. In our solar system, Jupiter currently moves on a slightly eccentric orbit, 5.2 times further from the Sun than the Earth.

Planteary orbits of our solar system. NASA

In our study, we considered scenarios with Jupiter orbiting between 4.2 and 6.2 times further from the Sun than the Earth, on orbits that varied from perfectly circular to moderately eccentric.

For each of the 39,601 systems we studied, we followed the orbital evolution of the planets for a million years – long enough for the Earth’s orbit to flex and tilt a number of times.

We were then able to calculate the rate at which our planet’s orbit varied from scenario to scenario, along with the amplitude of those variations.

Not quite ‘Rare Earth’ …

Proponents of the “Rare Earth” hypothesis have long argued that the origin and survival of life on the Earth is the result of such an unusual chain of circumstances that we are almost certain to be alone in the universe.

That viewpoint is based on the idea that the combination of factors that have made the Earth so habitable are so unlikely to occur as to be next to impossible.

At least in terms of the Milanković cycles, our preliminary results suggest that the Earth is not that unusual – it turns out that you can move Jupiter significantly without greatly increasing the amplitude or frequency of the periodic changes in the Earth’s orbit.

We do find a large number of planetary architectures that result in significantly greater swings in the Earth’s orbit than those we observe on our own planet. However, we also find that the majority of architectures tested result in orbital variations comparable to, or even less significant, than those we experience.

The maximum eccentricity of the Earth’s orbit, over a period of 1 million years, as a function of the orbit of Jupiter. The orbit of Jupiter in our own solar system is marked by the open circle. Whilst there are many regions where the Earth’s orbit is driven to high eccentricity (and therefore to significant climate variability), it is noticeable that there are also many solutions for which the variation in the Earth’s orbital eccentricity is lower than that for our own solar system. Results to be presented at the 13th Australian Space Science Conference

Our results are clearly only preliminary, but they do reveal one technique by which we could whittle down our list of potentially habitable worlds to help select the most promising targets for the search for life.

By the time we have found exo-Earths in a given system, we will have a very good handle on the existence of, and orbits of, the more massive planets in the system. It will then be fairly easy to simulate the orbits of those planets, to see how the exo-Earth’s orbit is nudged and tweaked over time.

This will allow us to work out which of those exo-Earths will suffer the greatest excursions in their orbits, on timescales of thousands and tens of thousands of years – allowing us in turn to target those that are the most “Earth-like” in our search for life elsewhere.

Join the conversation

18 Comments sorted by

  1. Brendan Smith

    Masters of Sustainability student at Monash Universiy

    Just wondering, with human technology and innovation- would it be possible or even sensible to terra-form Mars? And if so what effect would that have?

    1. Dale Bloom


      In reply to Brendan Smith

      Mars is not suitable for terraformation by species as we know them, and Mars can never have an atmosphere.

      Mars lacks a sufficient magnetic field to repel away solar winds, and the planet is being blasted with streams of solar radiation particles from the Sun, displacing any atmospheric gasses.

      It has been theorised that Mars did once have a greater magnetic field, (and an atmosphere), but the magnetic field decreased as the core of the planet cooled.

    2. Damien Westacott


      In reply to Brendan Smith

      Heh - take a read of Red Mars, Green Mars and Blue Mars by Kim Stanley Robinson. It's a great telling of the next few hundred years of history, centred on the exploitation of Mars.

      It doesn't just talk about the how - there's a whole lot about the why.

    3. Alex Cannara

      logged in via LinkedIn

      In reply to Brendan Smith

      Mars has no possibility for surface life as we know it, because it has no magnetic field to protect its atmosphere from solar-wind stripping. Without an atmosphere and magnetic fiield, surface life would be destroyed by solar & cosmic radiation.

      If we find Mars had organic life eons agon, it will be because its core had not yet cooled and lost its ability to generate a magnetic field.

      If we send a team to Mars, they'll go in the equivalent of a sphere of water with 15ft thick walls, land somehow with enough excavation gear to quickly burrow underground to build living quarters.

      If one tried to create an artificial atmosphere (as in the old Schwarzenegger flick), it would be fruitless. So plants, etc. for food would have to be grown underground in artificial light (like weed in Calif.), or via mirrors on the surface aiming light into artificial caves.

      But, at least power will not be a problem -- plenty of nuclear fuels like Thorium on Mars.

  2. Arthur James Egleton Robey

    Industrial Electrician

    Love the graphics.
    What is this fascination with life at the bottom of a gravity well?
    A video is worth ten thousand words. Good music too.

    The world's population doubles every 35 years. Soon I will see my second doubling. ie the for every person that was alive when I was born there will be 4. (Two doublings). And then the doubling stops? Maybe, but you will not enjoy the process.
    There is room at the LaGrange points for many orders of magnitude more people than infest this beautiful, sacred but finite orb.
    There is no acceptable alternative. Don't blather on about living sustainably. You missed the boat in 1982. (Limits to Growth Report stabilised world model.)
    For this to happen we need solid state nuclear, huge strides in robotics and 3d printing.

    Someones ego is going to be outraged. For them I prescribe di-methyl triptomene, DMT.

    1. Arthur James Egleton Robey

      Industrial Electrician

      In reply to Arthur James Egleton Robey

      Question what are serotonin analogues doing in mycelia? Why do mycelia have neurotransmitters?
      Hypothesis: Mycelium spores are protected by the hardest organic material know. Just what you need to exist millions of years in space.
      The mycelium colonised the planet long before plants and animals evolved here.
      Our inability to recognise intelligence in other species, blinds us to the existence of the mycelial mat beneath our feet and beneath the ocean bed.
      For those with outraged egos I prescribe psilocybin.
      Try and remember that the model is not reality. The map is not the terrain. Your ego will not disappear in a puff of smoke if its model is wrong.
      And the chances are that we have completely misunderstood the nature of intelligence on this planet.

    2. Paul Prociv

      ex medical academic; botanical engineer

      In reply to Paul Prociv

      Whoops - overlooked that they also require oxygen - was that around when they invaded Earth?

  3. David Maddern

    logged in via Facebook

    "Proponents of the “Rare Earth” hypothesis have long argued that the origin and survival of life on the Earth is the result of such an unusual chain of circumstances that we are almost certain to be alone in the universe.

    That viewpoint is based on the idea that the combination of factors that have made the Earth so habitable are so unlikely to occur as to be next to impossible."

    This forgets that it isn't that the environment is ideal for us, it is us that is perfect for the environment.
    It is a feature of sustainable life, natural selection. Therefore, a Goldylocks planet isn't necessary for life, so we are much more likely to find life than a Goldylocks planet.

    1. Jonti Horner

      Vice Chancellor's Senior Research Fellow at University of Southern Queensland

      In reply to David Maddern

      A good point -- there are quite a few reasons I think Rare Earth isn't the way to go - but I'm more of a "glass half-full" person than a "glass half-empty" one. I agree entirely that people sometimes get this argument back-to-front -- arguing we're only here because the Earth is perfect for life, when in reality, the Earth appears perfect for life because we have evolved to fit perfectly to what the Earth has to offer :)

  4. Daniel Cotton


    I logged in just to say the picture of the space craft used for illustration in the story is either from the film Forbidden Planet or looks a lot like it.

  5. John Nicol

    logged in via email

    Jonti Horner

    This is one of the most interesting articles to have appeared on The Conversation - factual if imaginary science without a hint of politics! Marvellous.

    It also touches on the many complex and very real influences which can affect our climate here on earth - in the long term as in the ice ages well known cycle, but with the suggestion of smaller more rapid oscillations which might also combine to form "beats" which have much less dramatic (than an ice age) but short term observable effects as recorded over the last years of our historical records.

    Welcome to Australia from the famous Halls of Oxford. Astrophysics I guess or the Clarendon?

    1. Jonti Horner

      Vice Chancellor's Senior Research Fellow at University of Southern Queensland

      In reply to John Nicol

      Thanks John :) It's a fun subject - I'm looking forward to getting more results out and figuring out what scale and periodicity you'd get for the Milankovic cycles in other planetary systems. Its a bit of a way off, but we'll get there in the end :)

      I was actually in Theory, on Keble road, when I did my D.Phil. - in the five person student office on the ground floor that looked out towards Keble college. Happy days :) Lincoln College :)

      I honestly don't think that there is any way that the Milankovic cycles can explain the current short term movement of the climate - it just doesn't fit. These cycles go on my longer timescales - although the beating idea is a cool one :)

    2. John Nicol

      logged in via email

      In reply to Jonti Horner

      G'day Jonti

      The brick patterns on Keble's walls would have made an interesting backdrop for a theoretician to think about.

      Yes, I understand that the Milankovic cycles are very long periods which lock in at 100,000 years etc, but I believed they had some relationship with the orbital periods of the major planets which implies a gravitational influence on the earth's rotational/orbital behaviour on top of the classical precessions of the eccentricity etc. as well as influencing the sun


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  6. Alex Cannara

    logged in via LinkedIn

    Nice article.

    Just don't bother trying to go to some "comfortable" exo-planet!

  7. Mark McGuire

    climate consensus rebel

    This could have been a great article if it included all the UN-IPCC climate science. Quote: "Speaking at the Planet under Pressure conference in London, Prof Will Steffen, a global change expert from the Australian National University, said that this period of climate change caused by humans, known as the ‘anthropocene era’, could ultimately cause the whole system of ice ages followed by warm periods, that has allowed life on Earth to flourish, to be over." Yes, you read that correctly. Milanković cycles – the periodic changes in the Earth’s orbit that are driven by our planet’s interaction with the other objects in the solar system, consequently glacial/inter-glacial periods, are over. Because of man made carbon(sic), so they say.

    1. Jonti Horner

      Vice Chancellor's Senior Research Fellow at University of Southern Queensland

      In reply to Mark McGuire

      The Milankovic cycles won't stop - but if the planet warms by enough, then the low points in the cycles won't be cold enough to trigger glaciation events. The only reason they do at the moment, as far as I know, is that the Earth's climate was sufficiently cool that the relatively small Milankovic cycles we experience could tip us between two equilibrium positions - one with slightly more flux, and less glaciation, and one with slightly less flux, and more glaciation.

      For the great bulk of the…

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