Within the astronomical community – and seemingly everywhere else in the world – it has long been believed that Venus is the planet closest to Earth. Even the likes of NASA have supported this conviction, in fact. Yet three scientists recently decided to reconsider the matter. And their findings may surprise you – because they have apparently upended the prevailing scientific consensus.
For some time now, of course, it has been understood that planet Earth is a part of a complex of bodies that orbits the Sun: the Solar System. Yet how the elements of the Solar System should be designated has been controversial from time to time over the years. Nevertheless, scientists have now reached a broad consensus. This states that the Solar System is made up of eight planets, five dwarf planets and other smaller entities.
It was actually the invention of the telescope that proved pivotal to mankind’s initial understanding of the Solar System. During the 17th century, for instance, Galileo Galilei was able to determine specific features of certain entities within the system using a telescope. He even noted that sunspots etch the Sun and that craters define the Moon’s surface.
And after Galileo’s findings, several new discoveries about the Solar System soon followed. Christiaan Huygens, for example, noted two moons orbiting Saturn as well as the shape of that planet’s rings. Meanwhile, Giovanni Cassini recorded four more moons of Saturn and the Cassini Division, a 3,000-mile-wide expanse between two of the planet’s rings.
Then in 1705 an Englishman by the name of Edmond Halley came to a massive conclusion. He noticed that a particular comet in space was in fact being recorded from Earth every 75 or so years. And this, he realized, was indicative of other cosmic objects orbiting the Sun in addition to the known planets.
And as time passed and scientific instruments developed, experts discovered that there are far more planets in the Solar System than simply Earth. Today, in fact, these are understood to include Mars, Venus, Mercury, Saturn, Jupiter, Neptune and Uranus. Another entity, however, has proven to be quite controversial within astronomical circles.
In 1930, you see, an American by the name of Clyde Tombaugh discovered Pluto. And at that time Pluto was considered to be the ninth planet of the Solar System. However, this assertion proved to be a point of contention among astronomers. This was particularly the case when astronomers later discovered other non-planetary objects of a comparable nature to the new planet.
Yet Pluto was for some time thought to be bigger in size than the planet Mercury. In fact, this was a belief widely held by astronomers for just under half a century. Eventually, though, it was established that Pluto’s mass is, in fact, only equivalent to somewhere around 5 percent of Mercury’s.
Still, by this measure, scientists considered Pluto to simply be the smallest planet in the Solar System. Yet in addition to having a relatively insignificant mass compared to the other eight planets, Pluto also possesses some other abnormal features. Particular examples of these related to the nature of the body’s orbit.
In the 1990s, then, astronomers started discovering other cosmic objects that in many ways exhibited similarities to Pluto. And it then began to seem as though these objects belonged within their own category of classification. Many scientists as a consequence thought that Pluto itself could no longer be considered to be a true planet.
Pluto is not the only “planet” to prove controversial, though. In 2005, in fact, astronomers first observed a planet-like entity known as Eris. This body was actually found to be some 27 percent larger than Pluto. Consequently, it was for a time apparently considered to be the tenth planet of the Solar System. But not everybody was ready to accept this.
At the time, you see, Eris was one of three discovered objects considered to be on a par with Pluto in terms of size. The other entities are known as Quaoar and Sedna. And as a consequence, astronomers came to the conclusion that they would need to rethink the consensus over the definition of a planet.
The International Astronomical Union is the organization responsible for classifying astronomical bodies and objects. It was formed in 1919 and is today headquartered in the French capital city of Paris. So in 2006 the group sought to end the controversy surrounding Pluto by officially laying down the law about what a planet actually is.
“A planet is a celestial body that is in orbit around the Sun,” the International Astronomical Union stated. “[It] has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. And [it] has cleared the neighborhood around its orbit.”
Under this definition, then, neither Pluto nor Eris could be considered to be a planet. Rather, the new criteria established by the International Astronomical Union saw these entities being classified as so-called dwarf planets. The other dwarf planets today include Haumea, Makemake and Ceres. In many ways similar to true planets, dwarf planets do, however, have some important differences from them.
So, just like true planets, dwarfs orbit the Sun and have an almost spherical shape. However, these planets fail to meet an important point that the International Astronomical Union set out: they have not “cleared the neighborhood around [their] orbit” in the way that planets have. They are also distinguishable in nature from moons since they don’t orbit planets.
Yet in spite of Pluto’s downgrade from planet to dwarf, the entity is still shrouded in controversy. Certain astronomers have spoken out against the reclassification, for example, claiming that it undermines the definition of some of the other eight planets. And even pockets of the general public have proven resistant, with petitions created to encourage a reconsideration.
If nothing else, though, the debate surrounding Pluto illustrates that established wisdom within scientific fields of study can be challenged. Indeed, it’s possible that supposed truths once considered infallible can be revised and reshaped. And such was the case recently after three scientists published their findings on which planet is Earth’s closest neighbor.
Given the nature of the planets’ orbits around the Sun, then, the distances between them are in a regular state of flux. Yet even so, conventional opinion has claimed that Venus should be considered the planet that is Earth’s closest neighbor. But is now actually an appropriate time for a reevaluation of this thinking?
Christened after the Roman goddess of beauty and love, Venus orbits around the Sun in less than 225 days. And as it happens, the planet goes around the Sun in the opposite direction to Earth. This means that if you could stand on the surface of Venus, the Sun would ascend in the west and go down in the east.
Venus has plenty of similarities in nature to Earth, though. Venus has even been referred to as Earth’s “sister” as a result of their comparable sizes and similar planetary compositions. However, there are also considerable divergences between the two, which mean that they cannot be thought of as quite the same.
For starters, the atmosphere on Venus is denser than that on Earth – and it is made up of more than 96 percent carbon dioxide. The atmospheric pressure of Venus at its surface is also 92 times greater than it is on Earth. For context, this would be similar to the pressure you’d experience 3,000 feet beneath the surface of Earth’s waters.
Venus is also the warmest planet of the eight found in the Solar System. And while it’s possible that Venus may once have been covered in oceans, the scorching heat it suffers means that this is certainly no longer the case. In fact, the surface of Venus can be recognized as a dry desert dotted with rocks.
So Venus is at best a half-sister, but is she the closest planet to Earth? Well, even NASA has referred to Venus as Earth’s “closest planetary neighbor” in a written piece before. And Venus does indeed get closer to Earth than any other planet. This proximity doesn’t last long, however, as we will see later.
Despite sometimes nearing Earth, then, Venus actually spends a lot of its time a great distance away, given that its orbit is in the opposite direction to our planet’s. Consequently, scientists have argued that we can’t accurately consider the planet be our nearest neighbor. But if Venus isn’t our closest acquaintance, which planet really is?
In March 2019 three experts named Tom Stockman, Gabriel Monroe and Samuel Cordner published an article in Physics Today. Here, the scientists claimed that it is Mercury that really spends the most time closest to Earth. According to the trio, you see, the methods used to measure distances between planets have some flaws.
The team write in the article, “By some phenomenon of carelessness, ambiguity or groupthink, science popularizers have disseminated information based on a flawed assumption about the average distance between planets.” And they continue, “Using a mathematical method that we devised, we determine that when averaged over time, Earth’s nearest neighbor is in fact Mercury.”
As you may know, Mercury is named after the Roman of god of commerce and communication. And once Pluto had ceased to be officially considered to be a planet, that left Mercury as the smallest planet in our Solar System. On top of that, its orbit around the Sun takes less than 88 days – the quickest of all the eight planets.
What’s more, given the craters that define Mercury, its surface does not appear altogether dissimilar to that of our Moon. In fact, Mercury’s looks might suggest that the planet has been dormant from a geological perspective for a huge amount of time. The planet struggles to hold on to heat, too, and temperatures there can vary massively.
So, with the things that are known today about Mercury, it seems quite astonishing that it could have been overlooked as our nearest neighbor for so long. Yet it appears that a wayward scientific assumption was responsible for causing just that. But what led to such an apparently significant misunderstanding of the small planet’s true nature?
Well, the three scientists explain in Physics Today that the distance between planets has traditionally been calculated based on each one’s distance from the Sun. For example, Earth is considered to be one astronomical unit away from the Sun. And Venus is thought to be about 0.72 astronomical units away from the same star.
So in this more traditional way of thinking, one would calculate the distance between Earth and Venus by taking the smaller measure away from the bigger. In other words, the distance between Earth and Venus would work out to be 0.28 astronomical units. As Stockman, Monroe and Cordner claim, however, this method is overly simplistic.
“Although it feels intuitive that the average distance between every point on two concentric ellipses would be the difference in their radii, in reality that difference determines only the average distance of the ellipses’ closest points,” the scientists wrote in Physics Today. “Indeed, when Earth and Venus are at their closest approach, their separation is roughly 0.28 astronomical unit.”
“But just as often, the two planets are at their most distant, when Venus is on the side of the Sun opposite Earth, 1.72 astronomical units away,” the scientists continued. “We can improve the flawed calculation by averaging the distances of closest and farthest approach… But finding the true solution requires a bit more effort.”
So the experts evidently realized that a more accurate answer would require a novel method. And the one that the scientists arrived at entailed measuring the distance between each spot on a given planet’s orbit against each spot on another planet’s orbit. Then based on this approach – which is known as the point-circle method (P.C.M.) – the team ran a simulation to test their thinking.
This simulation considered the Solar System and the planets that are contained within it. It measured the planets’ distances from one another not as they are right now, but as they would occur over thousands of years. And the subsequent results actually provided figures that differed from the previous numbers by some 300 percent.
Stockman, Monroe and Cordner therefore suggested that, in reality, Venus averaged out at some 1.14 astronomical units in distance away from Earth. Whereas Mercury, on the other hand, was notably closer at around 1.04 astronomical units away. But these weren’t the only interesting findings that the team suggested in relation to Mercury.
You see, the trio have actually implied that Mercury is not just the closest neighbor to Earth. Their calculations have in fact led them to make a claim that’s far bigger than even that. Indeed, they’ve suggested that Mercury is also the closest neighbor of every other planet in the Solar System.
The scientist team explained that their approach was completely new. They wrote in Physics Today, “As best we can tell, no one has come up with a concept like P.C.M. to compare orbits.” And they noted that it could have wider uses. They continued, “With the right assumptions, P.C.M. could possibly be used to get a quick estimate of the average distance between any set of orbiting bodies.”
The scientists ended the article with a use that they foresaw for their new method. They wrote, “Perhaps it can be useful for quickly estimating satellite communication relays, for which signal strength falls off with the square of distance.” Be that as it may, the scientists concluded, “In any case, at least we know now that Venus is not our closest neighbor – and that Mercury is everybody’s.”