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Life, Unbounded

Life, Unbounded


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An Abundance of Exoplanets Changes our Universe

The views expressed are those of the author and are not necessarily those of Scientific American.


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Earth-sized planets near and far (NASA/Ames/JPL-Caltech)

Planets in habitable zones, planets orbiting twin suns, miniature solar systems, rogue planets, planets, planets, planets. If there is one single piece of information you should take away from the recent flood of incredible exoplanetary discoveries it is this: Our universe makes planets with extraordinary efficiency – if planets can form somewhere, they will.

We’ve been sidling up on this fact for some time now, but it’s still a remarkable thing to acknowledge. Ten to fifteen years ago, as the first exoplanet detections began to come in, we understood that what we were seeing was potentially just the tip of the iceberg. These were massive objects (Jupiter sized or greater) and most of them were orbiting much closer to their parent stars than any equivalent giant planet in our solar system – hence the ‘hot Jupiter‘ moniker that is still used today. Statistics improved, as did our understanding of how detection techniques were biased towards finding these types of planets (owing to their greater gravitational influence on their parent stars), and estimates were made that suggested only a few percent of normal stars harbored such worlds.

Plot of exoplanet mass estimates versus year of discovery (generated from the online Extrasolar Planets Encyclopedia, thanks to Jean Schneider). The object shown in 1989 is known as HD 114762b, and is open to some debate in terms of actual discovery date and planetary classification as it may in fact be over 100 times the mass of Jupiter, nonethless it exists in this online compilation of exoplanetary data.

Of course time went by and astronomical instruments were refined, more and more data was accumulated, and longer orbital period planets and less massive planets were discovered. The figure to the left here illustrates the evolving range of planetary masses (or lower limits to planet masses) as a property of the year of discovery for confirmed exoplanets (excluding the thousands of to-be-confirmed-candidates from NASA’s Kepler mission). Here in 2012 we’re dipping well and truly into Earth-sized planetary terrain (about 0.003 times the mass of Jupiter on this scale).

By 2010 gravitational microlensing searches for planets were indicating that Neptune-sized objects on large orbits were at least 3 times more common that Jupiter-sized planets at similar distances from their parent stars. And hot on the heels of these measurements new Doppler, or ‘wobble’, detections of exoplanets indicated that at least 1-in-4 normal stars should harbor Earth-sized planets within about a quarter of the distance of the Earth from the Sun (0.25 AU).

It was becoming increasingly apparent that planets might be plentiful. Entering 2011 then the first big results from NASA’s Kepler mission began to make waves. With these came the statistical inference that the most numerous types of planets orbiting within 1/2 an Earth-Sun distance (0.5 AU) were Neptune-sized worlds, clocking in with a frequency of occurrence of about 17% (i.e. around 1 in every 6 stars). Close behind came Earth-sized objects, in about 6% of all systems. With a little extrapolation, and assuming a total of 200 billion normal stars in the Milky Way galaxy, it was clear that there might be millions of Earth-sized worlds in the habitable zones of their stellar parents, across the galaxy.

But things were just starting to warm up. The next item was another statistical inference from gravitational microlensing surveys, that now indicated a very substantial population of ‘rogue’ planets – giant worlds perhaps ejected from their stellar nests by strong gravitational interactions with other planetary chicks. The conclusion was that free-floating, wandering, objects as large, or larger than Jupiter, outnumbered stars in our galaxy by almost 2 to 1. It’s a remarkable result, but what about planets very much in the grasp of their parent stars, the equivalent of our own solar system?

Recently a new microlensing analysis by Cassan et al. appeared in Nature that explicitly targets planets orbiting between about 0.5 and 10 AU from their parent stars. The results solidify and carry forward all the measurements from before. About 17% of stars (give or take several percent) harbor Jupiter mass planets, cool Neptunes exist around about 52% of stars and Super-Earths (5 to 10 times the mass of Earth) exist around roughly 62% of stars. Even with sizable errors in these estimates (as much as 20-30%) the numbers are astonishing – there are at least 1.6 planets orbiting from 0.5 to 10 AU for every star in the galaxy. Combine this with the Doppler survey numbers (25% of stars with ‘Earth-sized’ planets within 0.25 AU), the Kepler numbers (17% of stars with ‘Neptunes’ orbiting within 0.5AU), and the microlensing estimates of 2 rogue giant planets per star in the galaxy and you have, well you have an awful lot of planets.

Of course one has to be careful in pulling these numbers together. Different detection methods and surveys have different biases, and if (for example) a giant planet orbits at 0.5 AU from its star then dynamical stability may preclude the possibility of other worlds nearby. Nonetheless, the bottom line is, I think, very clear; there really are planets everywhere, and they must number in the hundreds of billions in the Milky Way.

The results of glorious chemical and energy flux on a planet (L. Topinka, USGS)

Despite where we find ourselves, on a small rocky world, there was no reason to believe that the universe would make planets as efficiently as it seems to. Our situation is merely one data point, and a horribly biased a posteori one at that, and our models of planet formation are, to be quite frank, struggling to keep up with the flood of new data. Nonetheless, from the point of view of astrobiology and the search for life elsewhere, planetary bodies remain the primary, critical, target. There are simply no other environments in the cosmos that offer the same potential for diverse and complex chemistry in multiple phases of matter, and the potential for such long-term equilibrium (albeit a dynamic type of equilibrium with energy and chemistry in both sporadic and cyclical flux).

Thus, the sheer abundance of planets profoundly impacts the nature of our exploration of the universe and our quest to understand our own significance or insignificance. There is nothing trivial about the discovery of planetary plentitude, because it means that we are finally on the cusp of seeing whether a statement made two and a half thousand years ago is correct or not:

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow

- Metrodorus of Chios (Fourth Century B.C.)

It’s extraordinary to think how far we have come since these words were written.

(Oh, and as for moons, well don’t even begin to go there. Our solar system carries over 160 natural satellites around with it, so moons might yet turn out to be the most numerous planetary-type bodies of all…)

 

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 2:56 pm 01/20/2012

    While newly discovered non-luminous objects of mass do not appear to be so plentiful as to provide the identified missing mass thought to require the existence of still unidentified and undetected galactic dark matter (6-10 times to approximated mass of ordinary galactic matter), they should affect galactic gravitational evaluations. Perhaps the question of galactic rotation and the perceived requirement for galactic dark matter should be revisited.

    The requirement for galactic dark matter was primarily established by determining that the rotational velocity of disk objects did not comply with Kepler’s third law of planetary motion, diminishing as a function of radial distance.

    However, as Newton showed in his Principia, the accuracy of Kepler’s equations is dependent on orbitals that contain nearly zero mass (the Sun contains 99.86% of total Solar system mass) so as not to perturb each other’s orbits. The disks of spiral galaxies contain a relatively large percentage of total galactic mass; peripheral disk objects primarily interact with neighboring peer masses – they are self-gravitating. The motions of spiral galaxies can be generally described as rotating thin disks. Please see: Feng & Gallo, (2011), “Modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies”,
    http://www.raa-journal.org/raa/index.php/raa/article/view/858

    More precise measurements of 240 discrete Milky Way halo objects’ rotational velocities (including globular clusters, satellite galaxies and individual stars) physically constrains the potential location of any dark matter halo. Please see: G. Battaglia et al, (2005), “The radial velocity dispersion profile of the Galactic halo: Constraining the density profile of the dark halo of the Milky Way”, http://arxiv.org/abs/astro-ph/0506102

    While the disks of spiral galaxies do not comply with Keplerian rotational curves, this same data indicates that the rotational velocities of halo objects DO diminish as a function of their radial distance and that each independently orbit the distant galactic disk “like planets in the Solar system,” placing an upper constraint on the total galactic mass that could produce their observed velocities. Please also see: Bratek et al, (2011), “Keplerian Ensemble Approximation. The issue of motions of Galactic halo compact objects”, http://arxiv.org/abs/1108.1629

    These new findings, along with the questionable presumption used to infer the existence of galactic dark matter – that individual disk objects independently orbit some single dominating mass, should be enough to justify a rigorous reevaluation of galactic gravitation and redetermine whether any ‘missing’ mass might be required to produce observed galactic rotational characteristics.

    Link to this
  2. 2. Scienceangela 3:42 pm 01/20/2012

    According to the data, there are many many planets in the Milky Way alone. However, the question still stands: are we alone in the Milky Way Galaxy? The evidence so far seems to suggest that we are … is that true?

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  3. 3. caleb_scharf 4:06 pm 01/20/2012

    There is no real evidence one way or the other. Some people claim that the absence of any signs of ‘intelligent’ (i.e. technologically proficient) life is itself an indicator that such life is very rare, otherwise (the argument goes) there should have been enough time for it to come across us in some way. Personally I feel that there are many ways around that particular logic, from the briefness of existence of modern humans to the potentially intractable problems of interstellar travel that could severely slow and limit even advanced technologies from spreading very far, and never mind the assumed psychology/evolutionary imperative to spread out and/or communicate – which could be a dangerous thing to do (check out my earlier post on Bad Aliens and Memes).

    The only real way to know is to keep looking, and to keep making progress in planet hunting.

    Link to this
  4. 4. jtdwyer 7:29 pm 01/20/2012

    caleb_scharf: I agree – the ~100 years in which humans have had the potential for detection of signals from some distant technological society would have to be coincident with their potential for transmission, plus propagation delay. I think that alone make the probabilities for detection extremely unlikely. No matter how may planets might exist with intelligent life, they could all be stuck in ‘dark ages’…

    Link to this
  5. 5. em_allways_right 10:28 pm 01/20/2012

    Lack of proof is not proof.

    Link to this
  6. 6. hybrid 4:40 am 01/24/2012

    These findings doesn’t sit too well with a conviction that our moon for instance is a result of a collision by a large planet sized body.
    The chances of such an event tends to be extremely slim, so having it happen on such a large scale is even more unlikely.
    “The Dynamic Ether “, instead, proposes that the star grows from the inside out, and not by accretion of dust which magically transforms itself into a rocky planet with a nuclear kernel.
    With a Dynamic Ether in place the star builds from a nuclear kernel, overbuilds and casts off its excesses as planets, which in time, I guess, cast of their excesses to form moons . Apparently this is happening all the time, all over the cosmos.

    Link to this
  7. 7. caleb_scharf 8:14 am 01/24/2012

    The problem with alternative models for planet formation (i.e. other than the agglomeration of matter from a proto-stellar disk of gas and dust) is that they do not match our increasingly good astronomical observations of real proto-stellar/proto-planetary systems. We can now see much of the various stages of planet formation ‘in action’ in systems at different ages and circumstances, and while considerable uncertainties remain I think there is little wiggle room for any radically different process.

    Link to this
  8. 8. Laird Wilcox 10:40 pm 01/26/2012

    In a field of millet it’s possible (but unlikely) that none of them will grow, however that’s an extremely poor analogy in the first place.

    It’s encouraging to learn of the vast numbers of planets and moons. I have long felt that there was much less “missing” mass in the universe than the dark matter folks imagined. It simply hadn’t been found yet and now it is. There may be much more yet to be found, and none of it will be “dark.” “Dark” matter was simply a stab at trying to explain something that wasn’t understood yet.

    It’s quite possible that something called “life” in the form of microbes have existed on any number of planets briefly, but the extraordinary combination of events that created and THEN MAINTAINED life on earth could still be a one-time thing. Remember, not only does the planet have to be in the extremely unusual situation for life to form, but it also has to stay that way for billions of years — in our case over 4 billion.

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