5. Pluto-Sized Eris Rocks Solar System
In January 2005, Mike Brown and his team at Palomar Observatory, Calif. discovered 136199 Eris, a minor body that is 27 percent bigger than Pluto. Eris had trumped Pluto and become the 9th largest body known to orbit the sun.
In 2006, the International Astronomical Union (IAU) decided that the likelihood of finding more small rocky bodies in the outer solar system was so high that the definition "a planet" needed to be reconsidered. The end result: Pluto was reclassified as a dwarf planet. Pluto acquired a "minor planet designator" in front of its name: "134340 Pluto."
Mike Brown's 2005 discovery of Eris was the trigger that changed the face of our solar system, defining the planets and adding Pluto to a growing family of dwarf planets.
4. T. rex Tissue Dug from Bone
In 2005, Mary Higby Schweitzer and her colleagues reported in Science the discovery of what appeared to be soft tissues -– blood vessels, bone matrix and other cells –- inside the fossilized femur of a small T. rex.
Since then, the bones have revealed amino acids that resemble those of modern chickens, firming the link between dinosaurs and birds.
Schweitzer's discovery comes in a decade of other stunning revelations about the soft parts of dinosaurs.
In 2004, one of the few mummified dinosaurs ever found -- an amazingly well-preserved 66-million-year-old hadrosaur with intact, mostly mineralized skin -- was excavated from a ranch in North Dakota.
Then, in June 2009, researchers announced they had isolated molecules related to soft skin tissues from that hadrosaur.
3. Dark Matter's Existence Confirmed Directly
In the summer of 2006, astronomers made an announcement that helped humans understand the cosmos a little better: They had direct evidence confirming the existence of dark matter -- even though they still can't say what exactly the stuff is.
The unprecedented evidence came from the careful weighing of gas and stars flung about in the head-on smash-up between two great clusters of galaxies in the Bullet Cluster.
Until then, the existence of dark matter was inferred by the fact that galaxies have only one-fifth of the visible matter needed to create the gravity that keeps them intact. So the rest must be invisible to telescopes: That unseen matter is "dark."
The observations of the Bullet Cluster, officially known as galaxy cluster 1E0657-56, did not explain what dark matter is. They did, however, give researchers hints that dark matter particles act a certain way, which future research can build on.
2. New Human Ancestors Emerge
In 2002, researchers in northern Chad unearthed the 6- to 7-million-year-old skull of Sahelanthropus tchadensis -- known as Toumai. Only skull bones have been discovered, so it's not confirmed whether Toumai walked upright on two feet. But other Toumai remains make a stronger case that it greatly extends the human family timeline.
Then along came Ardi. In 2009, the nearly complete skeleton of Ardipithecus ramidus, a.k.a. “Ardi,” in northeastern Ethiopia bumped the famous “Lucy” as the earliest, most complete skeleton of a human ancestor ever found.
The 4.4-million-year-old Ardi could walk on two legs, but was also a skilled tree-climber. Her teeth suggest she ate many different types of food. And scientists theorize that males and females may have paired off at this time, significantly boosting survival, since females could intensify their parenting while males provided food.
If the studies prove true, Ardi marks the closest we have come to discovering the last common ancestor of chimpanzees and humans.
1. Alien Planets Seen Directly
The first alien planets -- called exoplanets -- were being detected in the early 1990s, but not directly. In 2000, astronomers detected a handful by looking for a star's "wobble," or a star's slight dimming as the exoplanet passed in front of it. Today we know of 400 exoplanets.
In 2008, astronomers using the Hubble Space Telescope and the infrared Keck and Gemini observatories in Hawaii announced that they had "seen" exoplanets orbiting distant stars. The two observatories had taken images of these alien worlds.
The Keck observation was the infrared detection of three exoplanets orbiting a star called HR8799, 150 light-years from Earth. Hubble spotted one massive exoplanet orbiting the star Fomalhaut, 25 light-years from Earth.
These finds pose a profound question: How long will it be until we spot an Earth-like world with an extraterrestrial civilization looking back at us?