What was Planet V?

About 3.8 billion years ago, the inner planets were bombarded with a cataclysm of asteroids. Could Planet V have been the cause?

  • The Late Heavy Bombardment (LHB) occurred about 3.8 billion years ago, during which time Earth, Venus, Mercury, and Mars were assaulted by asteroids.
  • Scientists are pretty sure the LHB occurred, but they're not certain what caused it.
  • It could be that a hypothetical fifth inner planet once existed in our solar system. As it left, it may have caused the LHB.

When the Apollo astronauts brought back rocks from the Moon, they observed an interesting shared characteristic. Many of the impact melt rocks they collected—essentially, the melted remains of asteroids or comets—seemed to have hit the Moon around the same time, about 3.8 billion years ago. Were it business as usual in space, one would expect to see a wide distribution of asteroids or comets that occasionally struck the Moon by sheer luck. But instead, it seemed like something catastrophic had happened all at once.

When scientists looked to other rocky objects in our solar system, they began to see evidence of a similar bombardment around the same time. Craters on Mars can be dated to between 4.1 and 3.8 billion years ago. Portions of the surface of Mercury, too, appear to have originated around that time, which is odd since the planet formed a few hundred million years before that. Researchers believe that this bombardment, in conjunction with volcanic activity, churned up Mercury's surface. There's even evidence on Venus and Earth that something incredible happened 3.8 billion years ago. Researchers now call this period the Late Heavy Bombardment (LHB).

At first, the LHB was a controversial idea, but as we began to see more evidence on other planets, it started to gain more acceptance. Clearly, something happened; the four terrestrial planets in our solar system bear thousands of scars, all originating from around the same time. But if we were under assault from the stars, then what caused it?

There are a few different hypotheses for what caused this event. It could be that Jupiter and Saturn ponderously shifted their orbits over time, upsetting the asteroid belt. Or, a large asteroid crossing Mars's orbit could have broken apart, sending fragments scattering across the solar system. But one of the more interesting explanations is that our solar system used to have another terrestrial planet: Planet V.

What happened to Planet V?

NASA

An artist's impression of the Late Heavy Bombardment.

Based on simulations of our solar system, some researchers believe that a rocky planet used to exist between Mars and the asteroid belt. It's possible that Planet V began with a fairly stable orbit, but, over a period of about 600 million years, the inner planets may have driven the planet into a highly eccentric orbit.

This planet would have been a quarter the mass of Mars, easily tossed around by its larger neighbors. Researchers have run hundreds of simulations, some of which tweaked the asteroid belt to account for the mass it would have lost during the LHB, the orbits of other planets, or made other changes to account for the possible differences between our solar system billions of years ago and today. From these simulations, we can see three possible scenarios if Planet V did in fact exist.

As Planet V's orbit became more eccentric, it could have crossed into the asteroid belt, scattering the asteroids to the four remaining inner planets, sending a surface-changing bombardment that may even have brought water or other chemicals to planets, including, possibly, our own. Eventually, it would have swung out of our solar system.

The second possibility is much like the first: Planet V's orbit became eccentric, and it disrupted the asteroid belt. Only instead of swinging wide into space, it slowly fell into the Sun.

Simulations also showed a third possibility: Planet V collided with another planet in our solar system. If this happened, though, there would be some massive crater on the surface of one of the inner planets. If Planet V was much smaller, it could have crashed into Mars, forming the Red Planet's massive Borealis Basin, which covers about 40% of its surface. Rather than dragging any asteroids out of orbit, the debris from the impact could be responsible for the LHB all on its own.

While it's a fascinating hypothesis, Planet V is still just that. We know that something caused the LHB. The evidence is scattered across the face of our neighboring planets. But what caused it—whether the movements of existing planets over time, the presence of a neighboring planet that moved out or fell into another celestial body, or something else entirely—is still a mystery.

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New fossils suggest human ancestors evolved in Europe, not Africa

Experts argue the jaws of an ancient European ape reveal a key human ancestor.

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  • The jaw bones of an 8-million-year-old ape were discovered at Nikiti, Greece, in the '90s.
  • Researchers speculate it could be a previously unknown species and one of humanity's earliest evolutionary ancestors.
  • These fossils may change how we view the evolution of our species.

Homo sapiens have been on earth for 200,000 years — give or take a few ten-thousand-year stretches. Much of that time is shrouded in the fog of prehistory. What we do know has been pieced together by deciphering the fossil record through the principles of evolutionary theory. Yet new discoveries contain the potential to refashion that knowledge and lead scientists to new, previously unconsidered conclusions.

A set of 8-million-year-old teeth may have done just that. Researchers recently inspected the upper and lower jaw of an ancient European ape. Their conclusions suggest that humanity's forebearers may have arisen in Europe before migrating to Africa, potentially upending a scientific consensus that has stood since Darwin's day.

Rethinking humanity's origin story

The frontispiece of Thomas Huxley's Evidence as to Man's Place in Nature (1863) sketched by natural history artist Benjamin Waterhouse Hawkins. (Photo: Wikimedia Commons)

As reported in New Scientist, the 8- to 9-million-year-old hominin jaw bones were found at Nikiti, northern Greece, in the '90s. Scientists originally pegged the chompers as belonging to a member of Ouranopithecus, an genus of extinct Eurasian ape.

David Begun, an anthropologist at the University of Toronto, and his team recently reexamined the jaw bones. They argue that the original identification was incorrect. Based on the fossil's hominin-like canines and premolar roots, they identify that the ape belongs to a previously unknown proto-hominin.

The researchers hypothesize that these proto-hominins were the evolutionary ancestors of another European great ape Graecopithecus, which the same team tentatively identified as an early hominin in 2017. Graecopithecus lived in south-east Europe 7.2 million years ago. If the premise is correct, these hominins would have migrated to Africa 7 million years ago, after undergoing much of their evolutionary development in Europe.

Begun points out that south-east Europe was once occupied by the ancestors of animals like the giraffe and rhino, too. "It's widely agreed that this was the found fauna of most of what we see in Africa today," he told New Scientists. "If the antelopes and giraffes could get into Africa 7 million years ago, why not the apes?"

He recently outlined this idea at a conference of the American Association of Physical Anthropologists.

It's worth noting that Begun has made similar hypotheses before. Writing for the Journal of Human Evolution in 2002, Begun and Elmar Heizmann of the Natural history Museum of Stuttgart discussed a great ape fossil found in Germany that they argued could be the ancestor (broadly speaking) of all living great apes and humans.

"Found in Germany 20 years ago, this specimen is about 16.5 million years old, some 1.5 million years older than similar species from East Africa," Begun said in a statement then. "It suggests that the great ape and human lineage first appeared in Eurasia and not Africa."

Migrating out of Africa

In the Descent of Man, Charles Darwin proposed that hominins descended out of Africa. Considering the relatively few fossils available at the time, it is a testament to Darwin's astuteness that his hypothesis remains the leading theory.

Since Darwin's time, we have unearthed many more fossils and discovered new evidence in genetics. As such, our African-origin story has undergone many updates and revisions since 1871. Today, it has splintered into two theories: the "out of Africa" theory and the "multi-regional" theory.

The out of Africa theory suggests that the cradle of all humanity was Africa. Homo sapiens evolved exclusively and recently on that continent. At some point in prehistory, our ancestors migrated from Africa to Eurasia and replaced other subspecies of the genus Homo, such as Neanderthals. This is the dominant theory among scientists, and current evidence seems to support it best — though, say that in some circles and be prepared for a late-night debate that goes well past last call.

The multi-regional theory suggests that humans evolved in parallel across various regions. According to this model, the hominins Homo erectus left Africa to settle across Eurasia and (maybe) Australia. These disparate populations eventually evolved into modern humans thanks to a helping dollop of gene flow.

Of course, there are the broad strokes of very nuanced models, and we're leaving a lot of discussion out. There is, for example, a debate as to whether African Homo erectus fossils should be considered alongside Asian ones or should be labeled as a different subspecies, Homo ergaster.

Proponents of the out-of-Africa model aren't sure whether non-African humans descended from a single migration out of Africa or at least two major waves of migration followed by a lot of interbreeding.

Did we head east or south of Eden?

Not all anthropologists agree with Begun and his team's conclusions. As noted by New Scientist, it is possible that the Nikiti ape is not related to hominins at all. It may have evolved similar features independently, developing teeth to eat similar foods or chew in a similar manner as early hominins.

Ultimately, Nikiti ape alone doesn't offer enough evidence to upend the out of Africa model, which is supported by a more robust fossil record and DNA evidence. But additional evidence may be uncovered to lend further credence to Begun's hypothesis or lead us to yet unconsidered ideas about humanity's evolution.