Unexplained Mysteries of 2013

Science is all about the pursuit of truth. New discoveries can provide answers, but they can also open the door to new questions.

Explore some of the mysteries left unsolved at the end of the year.

The Impossible Planet

Kepler-78b shouldn't exist. An Earth-sized planet with a rocky surface and an iron core, Kepler-78b is so close to its parent star that it completes an orbital revolution every 8.5 hours.

Kepler-78b is a curiosity because scientists have no way of explaining how the lava world could have formed given its proximity to its parent star.

The (Other) Impossible Planet

Exoplanet hunters had another mystery on their hands in 2013: a gas giant, HD 106906b, 11 times the size of Jupiter with a peculiar orbit. Unlike Kepler-78b, which is very close to its parent star, HD 106906b is 650 AU (astronomical unit) -- the measure of the distance between the Earth and the sun -- away from its parent star.

In fact, that's such a great distance that the orbit is larger than what astronomers once thought possible. Like Kepler-78b, the unusual distance means scientists do not yet how this exoplanet formed.

Mystery Date

Human ancestors were a promiscuous bunch. Ancient Homo sapiens mated not only amongst themselves, but also interbred with Neanderthals, a line of humans known as Denisovans, and a mystery lineage of humans. The unknown, extinct population isn't yet in the DNA record, as reported by LiveScience's Stephanie Pappas.

Given the different hominid species around at the time, Mark Thomas, an evolutionary geneticist at University College London, described it as a "Lord of the Rings-type world."

Hidden Hum

Do you hear that? That steady, droning, persistent sound that creeps at night.

If you are hearing things, you're not alone. Since the 1950s, reports have been coming in from around the world of people hearing what is known as "the Hum." As LiveScience's Marc Lallanilla reports, only about 2 percent of the population lives in a hum-prone area. It's louder at night than it is during the day, and typically heard in rural and suburban areas.

The Settlers

Christopher Columbus wasn't the first European to set foot in the New World. The Vikings preceded him. And in 2013, we learned of the arrival of a mysterious group of European settles to the "steps to the Americas" 300 to 500 years before the Vikings arrived in the New World.

Scientists had previously thought the Vikings were the first arrivals to the Faroes in the ninth century. Discovered at an archaeological site of Á Sondum on the island of Sandoy, researchers found evidence of human settlement in patches of burnt peat ash.

Although investigators have yet to discover clear evidence of the group's identity, possibilities include religious hermits from Ireland, late-Iron Age colonists from Scotland or pre-Viking explorers from Scandinavia.

Mona Lisa Mystery

Five hundred years after she had her portrait painted, we're still waiting to find out he identity of the woman in Leonardo da vinci's famous masterpiece, "Mona Lisa."

Lisa Gheradini Del Giocondo, the wife of a rich silk merchant, has long suspected of being the model behind Mona Lisa, and DNA tests of several skeletons found under a Florence convent will confirm whether one of them is Gheradini.

If one of the skeletons is Gheradini, who died in 1542, researchers plan on commissioning a facial reconstruction to determine any similarities between the skeleton and the portrait.

Read more at Discovery News