This Year's Biggest Discoveries in Science

Science mattered more than ever in 2013. Climate science questions raged after Super Typhoon Haiyan pummeled the Philippines. And scientific expertise figured in disarmament debates in Syria and in Iran's proposed halt to its nuclear activities.

Meanwhile, on the pure research front, investigators made plenty of intriguing discoveries in 2013. With plenty to choose from, and argue over, here's a top five list of some favorites from the year. And while you're at it, check out National Geographic's Year in Review.

1. Space gets more crowded. "Buy land, they're not making it anymore," Mark Twain famously advised investors. Twain never heard of exoplanets, of course. Caltech researchers suggested this year that at least 100 billion such worlds orbit stars in our Milky Way galaxy. That's a lot of new real estate. (See "Smallest Exoplanets Found—Each Tinier Than Earth.")

Of course, not all of them are places you would want to live. A November analysis from NASA's Keck Observatory team suggested that one in five stars may have Earth-size planets orbiting in their "habitable zones"—zones that could be friendly to surface oceans. A more recent climate analysis of habitable zones said that number may be too high, but that is still plenty of planets.

2. Human embryonic stem cells cloned.

After more than a decade of false starts, Oregon Health and Science University researchers announced they had cloned human embryos and collected stem cells from them. They also grew the cells into specialized skin and heart cells, a first step toward using them in transplant medicine.

The key to the team's success turned out to be the addition of caffeine to the cloning process. Now researchers will seek to discover whether these cells or similar "induced" stem cells, made without embryos, will have the most medical use.

3. Voyager reaches edge of the solar wind.

One of the year's biggest announcements came from news that actually happened in 2012. The aftershocks of a pair of solar storms in September confirmed that NASA's venerable Voyager 1 spacecraft had actually entered interstellar space. (See "Voyager 1 Leaves Solar System, NASA Confirms.")

"It is an incredible event, to send the first human object into interstellar space," study lead author Donald Gurnett, of the University of Iowa in Iowa City, told National Geographic.

NASA had long been hoping to announce that the far-flung spacecraft, launched in 1977, had passed the edges of the solar wind. Voyager 1's twin, Voyager 2, is also expected to soon reach interstellar space.

4. Mars lake looks hospitable to ancient life.

NASA's Curiosity rover continued to make historic tracks in 2013, finding that a vanished lake on the red planet could have supported life there more than three billion years ago.

The discovery is seen as vindication of NASA's efforts to look for past habitable conditions on Mars. The $2.5 billion rover next heads for Mount Sharp, in the center of Gale Crater, its original destination after landing. (See also: "Did Life on Earth Come From Mars?")

5. Lord of the Rings looking more like a documentary.

The human family tree suddenly sprouted some funky-looking shoots after a year of ancient DNA and fossil discoveries.

At the Dmanisi site in the Republic of Georgia, for example, researchers reported that what seemed like a lot of different-looking early human species likely were just one, Homo erectus. They based the claim on the discovery of a 1.8-million-year-old skull blessed with a mixture of more ancient and more recent characteristics. (See "Beautiful Skull Spurs Debate on Human History.")

On the genetic front, what looked like a Neanderthal bone in a Spanish cave turned out to actually possess the genes—the oldest DNA yet sequenced—of a different vanished early human species, the Denisovans.

Meanwhile, Siberia's Denisova cave, where Denisovan fossils were first discovered in 2008, yielded a toe bone that belonged to a Neanderthal woman from perhaps 140,000 years ago. (See "Ancient Incest Uncovered in Neanderthal Genome.")

This finding suggests that archaic humans mated with Homo erectus, as well as with some early modern humans in prehistory. A lot of modern people have a little archaic human in their genes, it turns out.

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