A newly discovered exoplanet is two-thirds the size of Earth and a relatively close 33 light years away.

But is it really a planet? UCF 1.01 is being called an exoplanet candidate. The reason is because a measured mass is needed to verify that it is a planet, but even the most sensitive instruments currently available are unable to measure exoplanet masses this small.

UCF 1.01 is so close to its star it orbits in 1.4 days. The planet's surface likely reaches temperatures of more than 1,000 degrees Fahrenheit and it probably has no atmosphere. Its surface may be volcanic or molten.

The researchers were studying a hot-Neptune exoplanet, designated GJ 436b, already known to exist around the red-dwarf star GJ 436, when data revealed clues that led them to suspect there could be at least one new planet in that system, and perhaps two. The team noticed slight dips in the amount of infrared light streaming from the star. A review of Spitzer archival data showed that the dips were periodic, suggesting that a planet might be blocking out a small fraction of light as it passed in front of GJ 436, as seen from Earth. 

Artistic rendition of UCF 1.01. Credit: NASA

"I could see these faint dips in the starlight and I wanted to determine their source. I knew that if these signals were periodic, they could be from an unknown planet," said
Kevin Stevenson, lead author of the study and a postdoctoral scholar at the University of Chicago. They sifted through hundreds of hours of observations collected from Spitzer, the Deep Impact spacecraft, the ground-based Very Large Telescope in Chile and the Canada-France-Hawaii Telescope near the summit of Mauna Kea in Hawaii. The transit technique relies on these tiny, partial eclipses to find exoplanet candidates.

Spitzer has performed science work on known exoplanets before, but UCF-1.01 represents the first time Spitzer has made a transit discovery.

With the finding of UCF-1.01, GJ 436 is likely now home to the first multi-transiting-planet system described by a mission other than Kepler. Of the 1,800 stars identified by Kepler as candidates for having planetary systems, only three are verified to contain sub-Earth size exoplanets.

The depth and duration of a transit reveals basic properties of an exoplanet, such as its size and distance from a host star. In UCF-1.01's case, its diameter is estimated at 5,200 miles, or two-thirds that of Earth, placing the world among the smallest on record. The team also noticed hints of yet another potential planet dubbed UCF-1.02, but its period was impossible to estimate.

Published in The Astrophysical Journal.