Among my earliest memories is dragging this huge Webster’s down on the floor before I could even read, just to look at the pictures of the solar system inside the front cover. Loved that stuff. Once I could read, I knew all the names, mean distance from the sun, rotational periods, known satellites, chemical composition of the planets, and so on. In the intervening years, approaching 6 decades, those parts of my brain have been repurposed or atrophied.
Which, it turn out, is probably all for the best. Much of what we thought we knew about planets has since been updated, in the sense of tossed. Since ca. 1960, way more moons, considerably different planetary compositions, even, I think, revised rotational periods for Venus and Mercury. And Pluto, according to some heretics and fools, demoted to mere planetoid.
Which is all good, except for that Pluto thing. We should hope that, as technology gets better, freeing more brainpower and providing more gadgets, we collectively should figure more things out. That’s the name of the game, right?
If only we – this is the collective we, which here necessarily means: those who report on this stuff, and those who then report on the reports – could remember the past enough to show a little reserve when announcing the latest findings, as if we’ve *finally* reached the bottom! We *finally* know How Things Are.
A formative experience in my youth was reading a paleo-anthropology book where the author took a moment to mention that all these theories about the origin of man were based on a collection of bones and artifacts that could easily fit on a kitchen table. This truth was reinforced and expanded later by Chesterton in the first few chapters of the Everlasting Man. Later still, read some anthropologist quip that never are the arguments so heated as when the the stakes are so low. It means nothing, practically, if A. africanus, say, is or is not in the direct line from apes to men – yet, somewhere, some anthropologist will go to the wall to defend or deny it.
Astrophysics and astronomy suffer this same problem, although it looks like, unlike the study of early man, the craziness is one step removed from the actual researchers themselves. Thus, we here that evidence of life has been found in the clouds of Venus – from people with journalism degrees. Digging just a tiny bit, what has happened is that absorption lines have been observes in spectra of certain wavelengths of light indicating the presence of the compound phosphine. In tiny, tiny concentrations. From this, via a series of impressive ifs and maybes, we get to life on Venus.
Spending a few more minutes googling around, it seems the real take away is: we know next to nothing about phosphine. It’s this very simple chemcal – 4 atoms – that exists on earth mostly as a byproduct of the metabolism of certain extremophile life forms. We think – nobody has yet figured out the extremely complicated biological processes that turn other phosphorus compounds in phosphine. Seems to happen, but, at this point, ‘by magic’ isn’t all that bad of an explanation.
The other place phosphine seems to occur naturally is on Jupiter. There, I’m assured, there are natural, non-biological mechanisms that explain its presence. Because who could doubt that we understand the chemical processes going on deep within a planet hundreds of millions of miles away that we’ve poked with probes a couple times?
What are the comparative chances that a) there’s life in Venus’ upper atmosphere, versus b) there are unknown non-biological ways phosphine can be generated? If there is life on Venus – and sending probes to capture some of her atmosphere and bring it back sounds like the way to go, Andromeda Strain being studiously ignored – way cool. But I’m not throwing a party just yet.
Another more interesting thing is the ever-changing theory of planet formation. Here, there’s nothing new in the news, just that I’m hearing about it now. For years, since at least as far back as when I was a kid looking at that dictionary, we were told about accretion discs, how, through random action and the conservation of momentum, clouds of dust and gas within a galaxy would form flattened rotating discs, out of which further random interactions would create a star. The radiation from this star would drive off the lighter gasses from its immeadiate neighborhood, leaving only heavier dust and chunks to accrete into small, solid inner planets. Far enough out from that new sun, the lighter elements had a chance to congeal into gas and ice giants.
Thus, using the clear arrangement of our own solar system as a template – as the only template available – theorists concocted a model that would require planet formation to follow the pattern we see locally. And this process was stated as if it where a done deal, most famously implicit in many guesses as to the values to be used in the Drake Equation: we were often told, by the likes of Sagan, that there are billions and billions of earth-like planets out there!
Then, starting about 15 or 20 years ago, astronomers had gotten the gadgets, time, and money needed to start seriously looking for planets around other stars. What they found: lots of gas giants very near their stars. Right where they shouldn’t ought to be.
To some extent, this is the predictable outcome of the methods used to find exoplanets. Giant planets very near their stars are going to be easier to find, generally, than small, rocky planets. So these preliminary findings – give it a century or two – are going to be biased away from ‘earth like’ planets and toward giant planets, generally, and particularly toward giant planets near their stars.
The problem: the traditional accretion disc theory really doesn’t have any mechanism to explain the presence of these near-sun giants. it’s tidy mechanisms are all geared toward the solar system we know best – our own.
So, I gather the theory of system formation has undergone a complete revamp. The most important feature: planets may be formed at some distance from their sun, and then, due to interactions with other bodies in their systems, change orbits, sometimes radically, sometimes getting launched entirely out of the system. Our local stable system might be a bit of a fluke.
The way it works, in my very amateur understanding: the star (or stars – many if not most systems appear to have multiple stars) has most of the mass in any system, and, over time, consumes more and more of the available matter in the disc. Everything is getting sucked towards the central star. But as often as not, matter – say, an asteroid or planetoid – may be accelerated, but miss the star. This is the dynamic in the well known ‘slingshot’ maneuver used to send earth satellites to the outer planets and on out of the solar system. You aim the probe at a planet, use the planet’s gravitational attraction to accelerate it. The planet keeps moving along its orbit, so the probe approaches only to have the planet move away. On net, the probe gains speed: its acceleration during the approach is greater than its deceleration upon moving away.
That net energy gain by the probe comes at the cost of a net energy loss by the body doing the accelerating. Mars, for example, looses a little speed every time we slingshot a probe past it. Its orbit shrinks just a tiny bit. The differences in mass make this deceleration negligible, but it is a real thing.
The new theories notice that this process, of capture, slingshotting, acceleration and deceleration, must go on all the time during a system’s formation, and, indeed, over its whole life. And that angular momentum will be conserved. Thus, if you imagine a messy cloud full of not only lots of gas but of many chunks of matter, you will have a situation where, as the larger bodies are formed, they are constantly accelerating smaller ones, lifting them to higher orbits or launching them right out of the system and maybe right out of the galaxy. And, as a result, the orbits of those gas giants are shrinking, moving them closer and closer to their star until, maybe, those planets themselves are consumed – or launched out of the system!
What our planet hunting may have found are giant planets nearing the ends of their lives (astronomically speaking), as they spiral ever down to their doom, to be consumed by their sun. On their way in, they would launch any smaller planets near the sun out into higher orbits or right on out of the system. Or consume them.
The last piece, the one we’ve known about since Newton: the bigger bodies interact with each other as well, most obviously in the way the planets orbit the sun, but also in affecting each other’s orbits. That’s how Neptune and (maybe) Pluto were discovered in the first place: the orbits of Uranus and then Neptune weren’t exactly where they ‘should’ have been. Astronomers were able to speculate exactly where another planet would need to be in order to account for the observed perturbations, and then looked. They nailed it to find Neptune; maybe to find Pluto (or it was a phenomenal bit of luck).
Now that we have this new, messy, theory that accounts for the new, messy appearances, how do we retrofit it to this nice, stable, life-sustaining system we happen to live in? One where the gas giants stayed put where they formed, more or less, and where at least 4 small rocky planets and innumerable asteroids *didn’t* get flung away?
To answer this, the scientists built a computer model. Of course they did. They put planets and other debris into different starting positions and let the simulation run to model billions of years, letting the big bodies accelerate the small and thus have their orbits shrink.
One interesting result – and always remember, models are an expression of prejudice, nothing more, until they have been verified against independent data – had the gas giants migrate in until Jupiter was about where Mars is now, then the pull of the further out gas and ice giants slowly pulled Jupiter back into the orbit it is in now. Another thing to remember: You get the results you tell the model you want. The simulations that showed something else were not discussed; I’d bet a dollar that, over time, the model was modified based on its own outputs so that it generated more ‘realistic’ outcomes. It was ‘improved’ in other words.
All that’s left is to find a way to test that model against reality. Let reality run for few hundred million years, and see how it compares to the models.
If you, like me, have long marveled at what an anomaly our beautiful earth appears to be, this current change in theories only makes our home planet more marvelous. Not only is it smack dab center in the habitable zone, with the most perfectly circular orbit of all the planets, with an inexplicable huge moon that drives the tides that make our planet much more friendly to life – but it has *stayed* in this position for at least several billion years! What? With all this cosmic tag and billiards going on, throwing planets around like rag dolls – and out little gem just got a pass? Extraordinary.
Finding ‘another earth’ is beginning to sound even less likely, Sagan’s billions and billions not withstanding. Maybe it does take a universe to have one earth.
The main point here, and one of the main points of this blog from its birth: one must always distinguish between what is in front of our eyes and the theories that purport to explain what is in front of our eyes. In this case, the right answer, the one that’s the right answer to almost all interesting questions: we don’t know. We don’t know how systems form. All theories have problems and attractive mechanisms in about equal measure.
And we’re having fun trying out our guesses.