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On Butterflies and Fish (Scott Tremaine)

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Image: Hubble Legacy Archive, ESA, NASA, Domingo Pestana

In many ways, the march of scientific progress can be viewed as an unrelenting peeling away of our innate sense of uniqueness. In the last few hundred years, we have gone from regarding ourselves as a divinely-favored species placed squarely at the center of a revolving cosmological light show, to a randomly evolved concatenation of DNA on an unremarkable planet, orbiting an average type of star, in a not terribly noteworthy position of a fairly run-of-the-mill type of spiral galaxy, in a gargantuan, and thoroughly indifferent, universe.

Such a precipitous descent from the privileged to the mundane is humbling, to say the least, but in what might be somewhat skeptically regarded as our innate predilection to the grandiose, many now have enthusiastically elevated our reluctantly-concluded averageness to nothing less than a cosmological statute: the Copernican principle, sometimes called the mediocrity principle, inverts the argument to pronounce that everything about our situation is overwhelmingly, necessarily, common.


Just as grabbing someone off the street at random will result, in nine out of ten cases, with coming face to face with a right-handed person, one should naturally expect that our planet, star, solar system, galaxy and particular region of the universe – taken, as it were, at random out of all the possible places we might have found ourselves – should instead be, well, pretty much the same as just about everywhere else.

But as fond as we are of postulating principles, modern science relies even more strongly on measurement. The philosophical pendulum might have swung from one end to the other, but now that we have the tools to carefully examine plentiful numbers of planets and planetary systems outside of our solar system, what does the experimental evidence say?

Scott Tremaine, Richard Black Professor of Astrophysics at the Institute for Advanced Study in Princeton, New Jersey, and an internationally renowned expert in both galactic-scale and planetary-scale astronomy, was an ideal person to ask. So, does the mediocrity principle hold sway, or does our solar system veered surprisingly towards the left-handed?

“Over the last two decades, we’ve discovered hundreds, probably thousands, of planetary systems orbiting other stars. We now know, then, that planets are common. This wasn’t at all obvious. In fact, in the late 19th century and the first half of the 20th century, the standard model for planet formation involved a very rare, unusual event: the close passage of two stars, which was likely to have only happened a handful of times in the galaxy. So, in that model, the solar system was this extraordinarily unique and unusual configuration that you practically never saw.

“That model had already been superseded in the 1960s, but we now know for certain that that’s wrong, because if you look at a typical star, even with our limited abilities to detect planetary systems around other stars, probably a significant fraction – 10%, 20%, 30%, depending on how you define it – have planets around them.”

Well, that seems to settle it. After all, if huge numbers of stars come with planets orbiting them, then that seems to give us a definite push towards garden-variety status.

But then, there’s this:

“Most systems we’ve seen don’t look like the solar system. Many of them have giant planets that are much closer to the host star compared to our own giant planets. In fact, many of them have planets that are much closer to the host star even than Mercury, the innermost planet in our own system.

“We don’t have a good theory for how those formed, and we don’t have a good theory for why they’re different from our own planetary system. One of the reasons for the difference, of course, is that it would be quite hard to detect our own planetary system.

“If we were sitting on a planet orbiting a star a few light-years away and were conducting the same planetary surveys with our current technology, we would just barely be able to see Jupiter. And we would be a little puzzled, because Jupiter is in a much more circular orbit than most of the analogs to Jupiter around other stars.

“So, astrophysicists sitting on this hypothetical planet ten light-years away would probably have said, ‘That system looks a little unusual because the planet that we see is a lot more circular than most of the planets that we see.’ ”

Score one for uniqueness, then. Perhaps we’re not so common, after all? It seems that the short answer is that it’s simply too early to tell. Given the fact that modern approaches to exoplanet detection are only 20 years old or so, it could well be that the statistics will level out as time, in tandem with detection technology, evolves. But then again they might not. Deciding between the two possibilities is simply an empirical question.

For Scott, however, there is a vital lesson to be learned here about transcending personal biases that extends far beyond the statistics of one particular solar system, however near and dear it may be to our hearts.

The goal of any scientific enterprise is to understand phenomena in the most general possible terms in accordance with the laws of nature. But in our quest to generalize, we often unthinkingly limit ourselves to our own particular experiences.

“The analogy I sometimes use is: suppose Darwin were developing the theory of evolution and all he’d ever seen was a butterfly. He might get the general idea of the theory of evolution right, but he probably would have put some things in it to explain why evolution could only produce things with thin wings of a certain size that fluttered around from plant to plant. We now know that, in addition to butterflies, there are lions and tigers and fish and birds; and that doesn’t mean that the basic idea of evolution would be different, but you have a much better idea, much better feedback, on how it must actually have worked, from the fact that you see this tremendous variety of systems.”

But, what if you don’t have the good fortune to be presented with such variety in the first place? What if we’re stuck in the dark, like Darwin and just his butterfly? What do we do then? That, says Scott, is the core intellectual challenge of astrophysics, what keeps him coming in to work each day with a smile on his face.

“This is an example of one of the things I enjoy about astrophysics: its detective nature – having to figure out to what extent you’ve been assuming that everything is the same as the things we’ve already seen. You have to have the imagination to ask if there are other things that are allowed by the laws of physics that we haven’t detected which might be quite different; and, if so, should we have seen them already, and are there techniques for detecting them?”

You also have to learn from past mistakes and lost opportunities, and the field of exoplanet astronomy has some lessons to teach us there, too.

“The irony, I think, is not so much that the technology is now good enough to detect exoplanets, it’s that these planets probably could have been detected much earlier, but the planets that you can see by these techniques are those that are quite close to the host star; and all the people who were looking for extra-solar planets in the 60s, 70s and 80s, when the technology was already good enough to detect them, thought that we should be looking for systems like our own solar system, in which case neither of these two methods would have worked.

“Because we’d only seen one planetary system, the community, then, not unusually made the assumption that all the other ones should look the same. As a result, nobody really took seriously the idea that you could use these techniques to deduce planets.

“Again, it comes down to the analogy of Darwin with the butterfly – no matter how well you understand evolution, if all you’ve seen is a butterfly, you’re not going to predict the existence of fish.”

We are all, to some extent, naturally limited by our pasts, unable to leverage first-hand encounters with the entire variety of potential systems. But the successful scientist can overcome the limitations of her past through a conscious act of creativity: harnessing her imagination to envision what might lie beyond her experiences, thoughtfully designing both theories and experiments that can capture the possible.

Such critical thinking skills may or may not be unique in the universe. But if we don’t exploit ours to the fullest, we’ll surely never find out.


For a complete list of Ideas Roadshow videos featuring Scott Tremaine, click here The eBook called Astrophysical Wonders containing the unabridged Ideas Roadshow conversation with Scott and a range of additional resources is available on Amazon .

Scott Tremaine is featured in the physics series Rethinking the Universe, which is now available on Vimeo On Demand. Check out the trailer which also includes Roger Penrose, Paul Steinhardt, Jill Tarter and Justin Khoury:

University staff and students may already have full access to all Ideas Roadshow resources on our Academic Portal through an institutional subscription, while select high school teachers and members of the general public have access to our School and Public Library Portals. For more information about institutional subscriptions, please contact


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