October 2013 was an exciting month for planetary science as researchers announced four unique planets. Nikku Madhusudhan of Yale University and colleagues found a rocky exoplanet that’s twice as wide as Earth and holds about eight times more mass than our planet. This super-Earth” orbits its Sun-like star, 55 Cancri, in just 18 hours.

Madhusudhan’s team determined the world’s density from its mass and size, and then used computer models to infer its characteristics. Astronomers know from previous work that the host star holds substantial amounts of carbon and little water ice or oxygen. The planet, known as 55 Cancri e, would have formed from similar material.

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth”, said Madhusudhan in his team’s October 11 announcement. “The surface of this planet is likely covered in graphite and diamond”.

A few days later, on October 15 2013 at the American Astronomical Society’s Division for Planetary Sciences meeting in Reno, Nevada, scientists with the Kepler spacecraft announced the discovery of the most compact planetary system yet.

The orbits of all five worlds would fit in one-twelfth of Earth’s distance from the Sun, and they all complete their journeys around their host star, KOI-500, within a handful of days. The star is cooler and less massive than the Sun.

Also on October 15, Meg Schwamb of Yale announced her team’s analysis of a world found by two citizen scientists involved with the Planet Hunters project. Named PHI , the planet is about 6.2 times Earth’s size, has a gaseous composition, and orbits a pair of stars in 138 days. While scientists have discovered such worlds orbiting binary systems, this one differs because another pair of stars circles PH l’s host pair with a radius about 1,000 times the Earth-Sun distance.

Then, in the October 18 issue of Nature, European Southern Observatory scientists described their discovery of a planet just 1.13 times Earth’s mass orbiting a member of the closest star system to us: Alpha Centauri B. This neighbor, however, circles its star in just 3.236 days — so it’s too close to its stellar host (4 percent of the distance between Earth and the Sun), and therefore too hot, to be habitable.

Xavier Dumusque of the Geneva Observatory in Switzerland and colleagues used the HARPs spectrograph attached to the 3.6-meter telescope at La Silla Observatory in Chile to break apart the light from Alpha Centauri B.

After filtering out noise, starspots, surface granulation, and 20 other factors from the star’s signal, they found a tiny fluctuation that they say corresponds to the slight wobble induced by an Earth-mass planet.

Because of the world’s proximity to Earth, scientists suggest that future observations look for light reflected or radiated from the planet, which could tell them about its surface and atmosphere (if it has one).

As planet-finding techniques become more precise, scientists continue to discover unexpected worlds — but one just 4 light-years away adds even more excitement.