Planets have a way of upending what we think we know about the universe. That can be upsetting. It might also be concerning to think that our current beliefs could be overthrown at some point. That’s just the way science works. Let’s use some discoveries about planets from the past, present, and future to illustrate this.
Past problems with planets
In ancient and medieval times, most educated people believed that Earth stood still in the center of the universe, and all of the heavenly bodies moved in perfect circles around us. But there were these five troublesome objects in the night sky that seemed to defy that regularity: Mercury, Venus, Mars, Jupiter, and Saturn. These are the five planets visible to the naked eye, and they seemed to wander around without conforming to the circle theory (in fact our English word “planet” comes from the ancient Greek word for “wanderer”).
Astronomers developed a model of the cosmos that tried to save the theory, by supposing the planets moved in circles on circles. Think of the double ferris wheel at the fair, where cars turn in circles on one arm, while that arm is attached to the end of another arm which is also turning in a circle. These circles-on-circles were called epicycles, and if you picked just the right size and speed of these epicycles, you could get them to produce the apparent wandering behavior of the planets in the night sky.
But ever more careful observations of these wanderers demanded more and more epicycles. Eventually in the sixteenth century, Copernicus suggested a simpler model, by putting the sun in the center of the universe. Earth became one of the planets orbiting the sun, and our vantage point of the third rock from the sun explained why the planets seemed to wander: every so often we passed the outer planets (Mars, Jupiter and Saturn) in our orbit, and the inner planets (Mercury and Venus) passed us.
Copernicus had to leave a few epicycles, though, because he was still committed to perfectly circular orbits. Kepler had even better data to work with around 1600, and he is the one who figured out that the orbits are ellipses rather than perfect circles. That was hugely significant for Isaac Newton upending what people believed about physics for about 1500 years.
Fast forward to 1781, when William Herschel discovered the planet Uranus. It is way out there—1.8 billion miles from the sun—compared to a mere 93 million miles for Earth. So it takes a long time for it to complete an orbit—about 84 years. By 1846, astronomers had enough data on its orbit to know there was something wrong: it didn’t conform to Newton’s laws of motion. That was a big problem, because Newton’s laws were amazing. They explained so much, they just couldn’t be wrong.
Maybe the problem with Uranus’ orbit was that we didn’t know all the relevant information. If there were another really big object somewhere in its vicinity, its orbit could be affected by that object’s gravity and get pulled out of the path Newton’s laws said it should take. A French mathematician named Urbain Le Verrier did a bunch of calculations and concluded that there must be another planet of such and such a size, located at such and such coordinates. The German astronomer Johann Gottfried Galle took those calculations and pointed his telescope at the coordinates. Lo and behold, there was the planet Neptune. This is one of the most spectacular confirmations that our theory about the way things work was correct… until it turned out not to be.
The same guy, Le Verrier, also knew there were some problems with Mercury’s orbit. It also didn’t quite match the path Newton’s laws said it should. So what do you do? Le Verrier did the math and predicted there must be another planet at such and such coordinates affecting Mercury. In 1843 they pointed their telescopes there and saw… nothing. Le Verrier gathered more observations and refined his calculations. He was so confident that he even named this hypothetical planet: Vulcan (I’m not making this up!). In 1859 they point their telescopes to the spot and once again saw nothing.
The problem with Mercury’s orbit turned out to be a major factor in showing that Newton’s laws of motion were wrong—well, at least incomplete. Einstein postulated with his general theory of relativity in the early 20th century that if you get close enough to a massive gravitational object like the sun, space itself gets warped and things don’t follow Newton’s laws.
So sometimes problems with a current theory lead to new discoveries (like Neptune); sometimes they lead to changing your theory (like general relativity). In both cases science shows itself to be fallible and trustworthy. Fallible, because what we believe on scientific grounds is always subject to revision in light of new discoveries. Trustworthy, because it is usually the enterprise of science itself that reveals the problems, refines existing theories, and suggests new theories. In the long run, science has a very impressive track record of figuring things out. In the short run, it’s not always clear whether our current theory is going to lead to more discoveries, or whether it is going to be overturned. For instance…
Present problems with planets
The big news with planets today is that there are a lot of them. Not around our star, where we’ve booted Pluto and its ilk out of the category and settled on there being just eight planets. But our technology has gotten good enough that we’ve discovered thousands of planets orbiting other stars. We’ve called these exoplanets.
One of the interesting observations about these exoplanets is the distribution of their sizes. You might think they would be evenly distributed from the small ones like Mercury, to the giant ones like Jupiter. But it turns out they are more clumped around certain sizes. There are quite a few in the Earth to Neptune range (Neptune is about 17 times Earth’s mass), and quite a few in the Jupiter range (Jupiter is about 95 times Earth’s mass). But there is a pretty big gap in planets in between those sizes.
That gap led to the core accretion model of planet formation. According to it, once you get a planet much bigger than Neptune, its gravitational pull is so big that it sucks up all the matter in the neighborhood and inevitably gets to be as big as Jupiter. (It’s more complicated than this, but that’s the basic idea.) But now that theory has some problems, because astronomers are discovering more and more exoplanets in that bigger-than-Neptune but smaller-than-Jupiter size range.
Like this guy, OGLE-2012-BLG-0950Lb. Its first name, OGLE, is a very cool acronym for an experiment that looks at things: Optical Gravitational Lensing Experiment. One of its middle names, 2012, is the year it was first discovered. But they had to wait six years to do some more observations on its star (the planet’s last name, BLG-0950L) and to determine the planet’s mass. It turns out to be about 40 times the mass of Earth—right smack in the zone we didn’t think was possible.
So what’s wrong? Is this just a rare exception, or is there something fundamentally wrong with the model? Or are our detection methods just fine-tuned to find the other sizes? The scientists aren’t sure yet. It’s a safe bet that future discoveries of more planets will help to sort it out. Here’s the scientific paper discussing all this with lots of technical data. Here’s a popularized version and more readable version with greater detail than I’ve given here.
Future problems with planets?
Maybe you’re not too worried about whether the core accretion model survives. Much more interesting to most people will be whether we ever discover life on any of these other planets. What would that do to your worldview?
There are scientific debates about the plausibility of life developing elsewhere. And especially when we’re talking about intelligent life elsewhere, the debates turn theological. I strongly recommend watching the video on this topic from the BioLogos conference last March with Stephen Freeland, Jennifer Wiseman, and Deb Haarsma. Deb has also been wondering about this topic in her recent piece, What would life beyond Earth mean for Christians?