The Border That Nobody Can Define
The most obvious question about the solar system is also one of the hardest to answer: where does it end?
One definition uses gravity. The Sun's gravitational pull extends so far that it can hold objects in orbit well beyond Pluto, in a region called the Oort Cloud. "It's gravitational bound to the sun. So it's in orbit around the sun. That's one way." But how far does that gravitational influence reach? "The gravitational pull is set to extend so far away that it's actually halfway in between our closest neighbor star, which is four light years away, Proxima Centauri."
Another definition uses radiation: the boundary of the solar system is where the solar wind, the stream of particles emitted by the Sun, loses the battle to cosmic radiation from the rest of the galaxy. Voyager 1 and 2, launched in the 1970s, have passed through this boundary. They measured it. But it did not happen at the same distance from the Sun for both probes: the boundary itself moves, shifting with the solar cycle. "It changes a bit, depending on... That's one of the reasons why it's not that very easy to say where the border is."
The honest answer is that the solar system does not have a clean edge. What the classroom poster depicts as a closed system ending at Neptune is a simplified picture of the part we know best and find most interesting.
What Is Actually in the Solar System
Most of the solar system's material is the Sun. Not most in the way that a dominant species might be called most: overwhelmingly most. "Most of the stuff, it's 99.9% of the whole material in the solar system is in the Sun."
Everything else, the eight planets, all their moons, the asteroid belt between Mars and Jupiter, the Kuiper Belt beyond Neptune, the Oort Cloud, every comet and dwarf planet and grain of dust, is 0.1% of the total. The solar system is, from a material standpoint, almost entirely one object.
Beyond Pluto there is the Kuiper Belt, an asteroid belt of icy objects in the outer solar system, and further still the Oort Cloud, a vast reservoir of objects left over from the formation of the solar system. We cannot observe it directly: objects that far from the Sun do not reflect enough light to be visible. We know it is there primarily because comets that travel toward the inner solar system on very long orbits have to come from somewhere, and the Oort Cloud is the best available explanation for where.
How the Solar System Came to Be
Before the solar system existed, there was a cloud of gas, primarily hydrogen and helium, with traces of heavier elements produced by earlier generations of stars. Something disturbed it, perhaps a nearby supernova, and in some regions the gas began to contract under gravity. As it contracted, it rotated faster, the way a spinning skater pulls in their arms. The material flattened into a disc around an increasingly dense centre.
At the centre, enough material accumulated to ignite nuclear fusion: the Sun. In the disc around it, dust and ice grains collided and stuck together. Over millions of years, these grew into planetesimals and eventually into planets. The planets in the inner solar system, closer to the Sun's heat, are rocky. In the outer solar system, where temperatures were cold enough for ice to exist in solid form, planets could accumulate more mass, pull in gas from the surrounding disc, and grow into the gas and ice giants: Jupiter, Saturn, Uranus, Neptune.
The solar system is, in this sense, the debris of a star formation process that ran 4.6 billion years ago. "But these theories, of course, we cannot go back in time to check it. So what we do is checking against exo planets around other stars and seeing if we can see similarities of very early planetary systems there."
Jupiter and the Question of Protection
Jupiter is by far the largest planet in the solar system, with a mass more than twice that of all other planets combined. Its gravitational influence shapes the orbits of many smaller objects. And for life on Earth, that influence appears to have been broadly beneficial: "Jupiter, the largest of the planets in the solar system, is somehow protecting against the inward coming asteroids and comets and so on because its gravity is much bigger than earth for instance."
The protection is not perfect, and the relationship is complex. Jupiter's gravity can also direct objects toward the inner solar system in some circumstances. But over the long history of the solar system, its dominant effect has been to redirect or absorb many of the large objects that might otherwise have struck Earth far more frequently.
The outer ice moons are another reason Mads watches the Jupiter and Saturn system closely. Both planets have moons with subsurface oceans of liquid water kept warm by tidal heating from their parent planet's gravity. Europa, a moon of Jupiter, is among the most discussed candidates in the search for life beyond Earth. What exists beneath the ice, if anything, is one of the most significant open questions in planetary science.
Why Pluto Is Not a Planet
In 2006, the International Astronomical Union voted to reclassify Pluto as a dwarf planet. The decision was controversial. It was also scientifically necessary.
The problem was not Pluto specifically. The problem was that beginning in the 1990s, astronomers started discovering other objects in the outer solar system of similar size to Pluto. Each new discovery raised the question of whether it should also be counted as a planet. If the answer was yes, the solar system's planet count would keep growing. The definition needed to be tightened.
The new definition required that a planet must orbit the Sun, have sufficient mass for gravity to pull it into a roughly spherical shape, and have cleared its orbital neighbourhood of other debris. That third criterion is what eliminated Pluto. It orbits within the Kuiper Belt, a region full of other objects. It has not cleared its neighbourhood. The other eight planets have. "All the other planets have done that."
The reclassification was not about Pluto getting smaller or less interesting. It was about the definition catching up with what astronomers had found.
The Solar System Is Not Special
Before 1995, we did not know whether other stars had planets. The solar system might have been unique. The first confirmed detection of an exoplanet orbiting a Sun-like star changed that picture, and the thousands of exoplanets confirmed since have changed it completely. "The sun is a very common star and the size of the planets. And we know that most stars have planets and more than one. So it's not that special, sadly."
What the solar system is, from the perspective of the broader galaxy, is a fairly ordinary example of a planetary system around a fairly ordinary star. What makes it extraordinary, from our perspective, is that we are in it. And because we are in it, we can study it in ways that are impossible for any other planetary system: we can send probes, land instruments, and take physical samples.
Mads is direct about what he is looking forward to next. Manned missions to Mars, which he believes are achievable within his lifetime. And missions to the ice moons: "Some of the moons around some of the bigger planets. Jupiter and Saturn has a lot of moons and some of them are ice moons, which would be really interesting to see if there's anything underneath the ice."
The poster does not show any of that. It shows eight circles in a row. The reality, as Mads describes it, is a vast, poorly bounded, mostly empty structure with one overwhelmingly dominant object at the centre, a handful of planets in the middle, and a great deal of uncertainty at the edges.