As society crumbles around us, we obliviously continue our tour of the Drake equation (which ironically makes more sense if you are wearing a mask):
N = R* × fp × ne × fl × fi × fc × L
In this equation,
- N is the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy
- R* is the rate at which new stars are created in our galaxy
- fp is the fraction of stars that have planets
- ne is the average number of planets per star that might support life
- fl is the fraction of life-supporting planets that actually develop life
- fi is the fraction of developed life that becomes intelligent
- fc is the the fraction of intelligent life that sends signals into space
- L is how long a signal-sending civilization survives and sends those signals
In the last post, after much rumination and hearty chortling, we decided R* = 2 Sun-like stars added to the Milky Way galaxy per year. Now we turn our attention to fp, the fraction of stars that have planets.
Planets are among the most effective ways to illustrate how bull-headed the human race can be. Most people have heard of Copernicus – more completely, Nicolaus Copernicus, who in the 16th century advanced the notion that the Earth and all the other planets in our solar system revolve around the Sun. Up until that time, the prevailing view espoused by Ptolemy all the way back around the year 150 A.D. – the idea that everything revolves around the Earth. Now – first of all, of course we all happily accepted the notion that everything revolves around us. We are dumb that way. But also of course, one of the driving points of this series of blog posts is to illustrate just how ordinary we are, and it turns out the universe agrees – so the Earth and all the other planets in our solar system revolve around the Sun. It wasn’t just that we hadn’t thought of such a scenario, either. Aristarchus of Samos – in the 3rd century *B.C.* – had already figured out how things really work. But it just doesn’t stroke the human ego enough, and so yada yada yada, Roman Empire rises and falls, Dark Ages, witch burnings, and boom, Copernicus re-figured it out almost two millennia later.
Regardless of our abject confusion about what they revolve around, we’ve known since ancient times that the planets are different from all those other points of light we call stars. The stars are ridiculously far away, and so even though they are in fact traveling at breakneck speeds through space, they look like they are standing still during the extent of any human lifetime. But the planets in our solar system – even though they are many millions of miles away – are far closer than the stars, and so we can actually see them move. We are also moving, being on a planet of our own – and the end result of all that motion is that the planets appear to wander about the sky from our perspective – and in fact that is exactly what the word “planet” means – wanderer.
Five planets can be seen with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. We have therefore been watching these little critters for quite some time. After Copernicus literally changed the world, the brilliant astronomer Galileo Galilei took the next steps – building not just on Copernican theory but also on the work of a Dutch spectacle maker Hans Lippershey who designed the first telescope. After creating his own telescope, Galileo showed us illustrations of the planets we had never seen, making them real worlds in our view for the first time. As telescopes advanced, more planets were discovered – Uranus, Neptune, and Pluto.
In the 1960’s, not long after we had first sent spacecraft into Earth orbit, we began sending them to the other planets in our solar system. In the 1970’s, we landed a couple on Mars (the Viking missions). In the 1980’s, the two Voyager probes toured the giant outer planets on their way to interstellar space. In the 1990’s, we began to detect planets around other stars – and since then, we have discovered over four thousand of them. We have quickly gone from wondering if other stars had planets to realizing that most of them probably do.
In fact we have also discovered a vast array of other small worlds that orbit our Sun. We’ve known for some time about the asteroids that orbit between Mars and Jupiter – and some of them are quite large. But there is also an unthinkably large (and sparse) cloud of little planetoids orbiting far beyond Pluto. The sum result of these findings, as well as the discovery of Pluto’s moon Charon which is not much smaller than Pluto itself, led to the demotion of Pluto to the status of “dwarf planet” in the 2000’s. This upset a lot of people, of course (and probably had no effect whatsoever on Pluto) – but the point really should be this: the solar system is simply loaded with large objects, many more of which are planet-sized than we had ever known. The discovery of thousands of planets beyond our solar system only underscores the point: worlds are probably quite common in our galaxy. So how does that come to be?
Thanks for asking. When we last left our fledgling star in the previous post, it had just achieved a state of zen, where gravity was causing it to collapse on itself, but nuclear fusion was generating enough energy to push back. We had been viewing the whole process as a shrinking ball of gas, but things are always a bit more complicated than that. The atoms that steady accumulate in the formation of a star aren’t all headed toward its center when they first start to get pulled in – they’re generally moving in all kinds of directions, which leads to a big swirling mess. Over time, just as gravity is pulling the star in on itself, it steadily kneads and flattens the swirl into a disk of material spinning around a single axis. You know how Saturn looks, right, with those rings spinning around a big ball? Well, most infant solar systems probably look something like that too – although the disk is spread out over a far greater distance than Saturn’s rings – so imagine huge rings spinning around a tiny ball. Eventually, just as the clumpy universe led to the star, clumps in the disk often accumulate into planets, until the planets eat up just about all of the disk, and you never knew there was a disk in the first place.
We understand this process fairly well – and now that we’ve seen thousands of worlds out there… wait a second, how do we know there are thousands of worlds out there? Well – there are a few ways to find them. In 1992, the first planet outside our solar system (also known as an exoplanet) was discovered orbiting a pulsar. Pulsars are the remnants of incredibly massive stars that ended their lives in equally massive explosions called supernovas. During those final death throes, these stars collapse at an incredible rate – and just as Carl Sagan so eloquently explained in his Cosmos series, like a figure skater pulling her arms in and spinning ever faster, these stars do the same thing, until they are spinning many times a second. As they spin, they emit pulses of radiation that we can detect on Earth – leading to their ingenious moniker “pulsar”. Pulsars spin at a constant rate, but in 1992, we noticed some subtle deviations in that rate for the pulsar affectionately named PSR B1257+12 (which I believe is also the name of Elon Musk’s next child). Aleksander Wolszczan and Dale Frail determined that the deviations were caused by one or more planets orbiting the star. It turns out this particular pulsar has three planets – and since then only four other planets have been found around pulsars. The rest have been found around relatively more “normal” stars. The dominant methods for detecting planets include the transit method – where the star’s light dims ever so slightly as a planet crosses in between the star and Earth – and the radial velocity method (or “wobble” method), where astronomer’s look for Doppler shifts in the star’s light (similar to the shift in sound that causes a train to seemingly drop pitch as it races by).
Ok – so we understand quite well how planetary systems form around stars, and we have observed thousands of them over the course of just a couple of decades. So it would seem that the piece of the equation we are after here – the fraction of stars that have planets – is probably pretty high. Multiple studies have suggested there are exoplanets around every star and/or that there is at least one planet for every star in the galaxy. So it is probably fair to say that fp is very nearly 100% (also known as a fraction of 1). Let’s go ahead and account for those lonely stars that may pop up here and there with nary a planet to be seen – and conservatively say that fp = 0.99. We have made yet another baby step of progress on our equation:
N = 2 × 0.99 × ne × fl × fi × fc × L
In the next post, we will take a closer look at ne – a guess at how many of all these planets are capable of supporting life. In the mean time, a toast to all you crazy planets out there – may you never stop spinning.