What If There's a Planet Between Mercury and the Sun? | Unveiled

VOICE OVER: Noah Baum WRITTEN BY: Caitlin Johnson
The universe is so big that it's easy to forget there are plenty of things we don't know about even our own solar system! Most sci-fi fans know about Vulcan, a hypothetical ninth planet orbiting the sun even closer than Mercury... But what would happen if it was real?? In this video, Unveiled imagines how different our lives would be if there was another planet orbiting extremely close to the sun...

What If There is a Planet Between Mercury and the Sun

The universe is so big that it’s easy to forget there are plenty of things we don’t know about even our immediate surroundings. The idea that there might be undiscovered objects hidden within our own solar system has prompted many hunts for missing or additional planets. When it took us so long to discover a planet as big as Neptune, is it possible there are yet more to find?

This is Unveiled, and today we’re answering the extraordinary question; what if there’s a planet between Mercury and the Sun?

As well as being the home planet of Spock, “Vulcan” is a hypothetical ninth planet orbiting the sun even closer than Mercury. Named for Vulcan, the Roman god of fire and equivalent of the Greek god Hephaestus, this planet would be extraordinarily hot if it existed. In the 19th century, many people were completely convinced that Vulcan existed and that if we just tried hard enough we would be able to observe it making a transit across the sun. And it wasn’t a fringe theory, either; the planet was named by Urbain Le Verrier, a Frenchman who was, at the time, the most renowned astronomer in the world. In 1846 he became a household name for discovering Neptune by studying irregularities in the orbit of Uranus. His mathematical predictions of Neptune’s existence and orbit were soon confirmed by telescope. Using the same methodology to study Mercury, Le Verrier decided that there must be another planet perturbing Mercury’s orbit.

When an amateur astronomer contacted Le Verrier in 1859 about what he believed to be the first Vulcan transit, Le Verrier wasted no time in announcing Vulcan to the world. People frantically began studying the sun, because if a planet was so close to it, it would orbit every few months, giving us quite a lot of opportunities to see it. But none of the sightings were credible and many were dismissed by other astronomers. The Vulcan theory was finally disproved for good in 1915 when Albert Einstein published his Theory of General Relativity. Until this point, the old model of Newtonian gravity suggested that there was a problem with Mercury’s orbit. Einstein’s new calculations managed to prove that Mercury was orbiting exactly as expected without the interference of an additional object, ending the hunt for Vulcan for good.

So, what were these astronomers observing when they thought they were seeing a new planet? The most obvious explanation is asteroids and comets. But interestingly, this region of the solar system is surprisingly empty. People have blamed “vulcanoids” for anomalous sightings of the planet, but vulcanoids are also hypothetical. Since their existence was first postulated, none have been observed. This doesn’t mean there are no objects at all. Due to the Sun’s glare, we’d only be able to observe them from Earth during twilight or solar eclipses. They may also be quite small - between 330 feet and 3.7 miles in diameter. Any larger, and we would have seen them. Mind you, there ARE asteroids that pass between Mercury’s orbital path and the Sun; but they also don’t orbit exclusively in what’s called the “Vulcan zone”. The reason we’re interested in vulcanoids isn’t just to find out what the earlier sightings were, but also to better understand the formation of the solar system. It’s worth noting that vulcanoids would be similar phenomena to trans-Neptunian objects - and it took us until 1992 to discover the second TNO after Pluto. With this in mind, we shouldn’t be completely discouraged from continuing the search.

But another big question is, if there really aren’t any vulcanoids, why not? In other star systems, we’ve observed exoplanets closer to their star than Mercury is to ours, making the “Vulcan zone” between Mercury and the Sun strangely empty. Scientists have been trying to solve this mystery, too, and one of the best explanations is solar wind. Theoretical studies have shown that planetesimals of all different sizes get pushed further out from the Sun over time. While it can take a few million years depending on their size, the solar system is 4.5 billion years old, plenty of time for the sun to push objects away. As larger objects were pushed out over time, smaller objects would have been pulled in by the sun’s gravity and burned to a crisp.

If there were a planet in the Vulcan zone, the solar wind alone could render it uninhabitable. Without a magnetosphere strong enough to sustain a protective atmosphere, it probably wouldn’t be home to an alien race. But there are more issues with being so close to the sun, namely, the temperature. Mercury already is scorching hot and practically desolate, and we’re talking about a planet even closer to the Sun. Though, interestingly enough, Mercury DOES have ice deposits at its poles. This discovery surprised many people, who thought the planet was too hot all over to sustain ice, but Mercury’s deep craters shield the ice from the harshest temperatures. Meanwhile, Venus, also closer to the Sun than Earth, is too hot for other reasons; its thick atmosphere traps sunlight in a runaway greenhouse effect. There’s no reason why this couldn’t also happen to a planet in the Vulcan zone, even if its poles, like Mercury’s, are comparably moderate.

And moderate poles will do nothing to protect Vulcan from solar radiation. Solar radiation is already a problem on Earth, contributing to all kinds of cancers, and gets even deadlier the closer you go to the sun. A thick atmosphere could cause another debilitating greenhouse effect, while a thin one wouldn’t do enough to protect from radiation and solar winds. Humans could potentially travel to this planet one day if our technology improved – though currently, high levels of radiation are enough to stop even our robots from functioning. Robots used to probe the depths of the Fukushima nuclear power plant broke down in a matter of weeks as they investigated. This doesn’t bode well for Vulcan’s possibility of producing life of its own – at least, producing life as we know it. It is potentially possible for it to produce life as we don’t know it, however, in the form of organisms resistant to the constant bombardment of solar radiation. Already on Earth, we have creatures that have naturally developed radio-resistance, ranging from plants to insects. Minas Gerais in Brazil, for instance, has many natural uranium deposits, and wildlife in the region has been shown to have radio-resistance. And there are also common creatures like tardigrades, which are capable of withstanding doses of radiation below 5,000 Grays; just 5 Grays is enough to kill a human within two weeks.

There’s no telling what kind of bizarre aliens would be capable of living in an environment like Vulcan. It could be host to microbial life such as tardigrades under the right conditions, but complex organisms like Spock would probably be too much for it to handle.

Ultimately, the sun’s solar winds are repulsive enough that large bodies get pushed out and small objects get pulled in, meaning that any planet unlucky enough to find itself in this dangerous domain would eventually be ejected or destroyed. And that’s what would happen if there was a planet between Mercury and the Sun.