Searching for that elusive exo-Earth

In the nearly 20 years since Michel Mayer and Didier Queloz confirmed the first known exoplanet around 51 Pegasi, the number of known exoplanets has risen to over 1860 – and there are more to come. The Kepler space telescope, before being hobbled by mechanical failure, created a massive database of planet candidates orbiting the 150,000 stars it looked at – some 4,175 in fact – which are still being checked. Eight new planets were confirmed just last week.

We can be sure there are a lot more out there. Kepler scanned just 0.28 percent of the sky in the direction of Draco, out to 3000 light years. In that patch, it could only detect planets whose orbits cross the disk of their star from our viewpoint. Other planetary systems, tilted at different angles, aren’t detectable by the transit method. But they will be there. And now the hot question – how many planets are like Earth?

Simulated Exo-Earth. A picture I made. Apart from the fractal artefacts, does anybody notice what's wrong with it?
Exo-Earth. A picture I made. Apart from fractal artefacts, does anybody notice what’s wrong?

Astronomers have found a few planets Earth-sized and below – including two of last week’s confirmations. Some are in the ‘Goldilocks’ orbit where the star’s warmth would allow liquid water to flow on a planetary surface. Though bear in mind that an observer using Kepler to scan our solar system would classify Venus as “Earth-sized” in the habitable zone. The problem is that transit-detection gives us diameter and orbital period, hence mass and density of the planet (and of its parent star). But it doesn’t give visual data – we can’t do spectroscopy on the atmosphere, for instance, though that’s possible with other techniques, and some data has been fielded about planetary atmospheres.

However, it’s only a matter of time (and money) before instruments are able to pick up more data from subtle fluctuations of stellar light. A photon here, a photon there – literally. From that, we’ll learn about planetary colour, atmospheric composition (via changes to starlight passing through it). Maybe we’ll learn whether any have large moons, if the orbit of that moon is in line with the star. Though I wonder. We’re looking for another Earth – but who says our world has been replicated?

Neptune. A picture I made with my trusty Celestia installation (cool, free science software).
Neptune. A picture I made with my trusty Celestia installation (cool, free science software).

One of the types we’ve found is the ‘hot Neptune’ – a world maybe twice the diameter of Earth with eight or more times the mass. About 19.3 percent of exoplanets found so far fall into this category, as opposed to 5.3 percent of Earth-sized worlds. They also orbit relatively closely to their stars. This is largely a function of technical limits – we can detect the bigger worlds more easily, and picking up the orbits of worlds that are distant from their stars requires years-long observations. So these proportions will likely change. But for the moment that’s where the data points.

Close to its primary, such worlds could be water planets, rather than the ice giants we have in our solar system. Maybe these ‘exo-Neptunes’ define ‘normal’. Or maybe every world is unique – product of many variables, obeying the same laws of physics but emerging in variations defined by subtle differences in composition, size, ambient temperature, and so on. Check out Jupiter’s biggest moons – all different, all formed in the same place at the same time.

The realities of physics mean we won’t travel to these exo-worlds any time soon. Or later (and yes, I know about the ‘Alcubierre drive’). But it’s fun to speculate…and I have a question. Suppose we found another Earth and arrived, en masse. Do you think we’d ruin it, the way we’re making a good job of ruining the Earth we’ve got? Just wondering…

Copyright © Matthew Wright 2015

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18 thoughts on “Searching for that elusive exo-Earth

  1. Read a Wall Street Journal article last week about the probabilities of our galaxy forming after the Big Bang, our planetary system forming and life evolving here – so tiny as to be nearly improbable. There are so many limitations now put on planets before they could be habitable by our species as to be equally improbable. Maybe that’s why SETI has never heard anything?

    1. I’d be very surprised if an alien Earthlike planet was inhabited by human-like aliens. There are so many variables. In absence of data, of course, the scientific answer comprises two words – ‘nobody knows’… 🙂

  2. The present state of physics notwithstanding, I think we’ll eventually discover a supraluminal drive. Call it an article of faith with no basis in present reality if you like. That’s literally true.

    It may be a good thing. I don’t think humanity, as a whole, has reached anything like the state of civilization required. I cite the present sorry state of the US Congress as proof. As an American, it pains me to write that.

    But I long to be a Starfleet officer, with the hope of a ship of my own someday, and go where no man has gone before.

    1. I agree – just now humanity isn’t morally fit to venture forth. We’d embarrass ourselves or break things. But, damn, it’ll be good when, as a species, we lose the ‘tude and get the tech! Like you, I think ways will be found in a while, and probably without invalidating Einstein.

  3. I wonder why we want to find another planet that somewhat resembles Earth. Wouldn’t it be more satisfying to our egos to believe that our planet is unique? Do we want to make friends with the inhabitants of that other planet? (That seems to me the most positive motivation.) Do we want to colonize it? Exploit it? Inhabit it? Beats me.

    1. Good point! Part of the answer may be in the fact that when we imagine what is out there, we always seem to imagine a reflection of ourselves. It seems to be what we look for.

  4. The good thing is human probably won’t be able to arrive en masse….maybe those that do go first will take some knowledge from here and be able to flexible enough to see the new planet as alike, but different – and allow it to be what it is?

    1. Hopefully! A few sensible people who can understand! En masse I fear it would be different. Humanity has this uncanny knack of breaking environments when we get going. Richard Attenborough apparently referred to us as the ‘scourge of the planet’. When I look around at the mess we’re making of Earth I suspect he’s hit the nail on the head. Luckily interstellar travel isn’t possible for us just now…

  5. This is facinating information. I’ve been studying early American colonization and thinking in parallel about what would happen if we discovered a distant inhabitable planet. I think it would be very interesting to investigate. I’m not sure we would do anything that different than Europeans did when they first came to the Americas. I don’t think we’ve grown that much.

    1. I think most colonial societies around the world are framed, in some way, by the framework of thinking when they were founded. That’s certainly true of New Zealand, as I’ve shown in my professional work on the history of the place; the ‘mythologies’ of the settlement – the aspirations, ideals and hopes – absolutely framed the way things panned out. These were typical of the nineteenth century ‘Pacific Rim’, and shared with the ideals of those expanding westward across the US to California at a similar time. And I think that’s true of other places too, which suggests that if or when we eventually manage an interstellar colony, that too will likely reflect whatever ideals or hopes it was founded around – and for some extended time after the colony has matured.

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