Scientists measure the size and mass of an exoplanet to determine its density and identify what it is made up of - solid, liquid or gas.
Exoplanet Kepler-138b, whose size is known to be similar with planet Mars, can now be further studied as its mass has also been determined.
Kepler-138b is among the four smallest exoplanets whose sizes and masses have been measured. Of these four, three are Kepler-138 exoplanets, the other two being Kepler-138c and Kepler138d. Kepler-138b is also the first exoplanet smaller than the Earth to have its size and mass measured.
With its size, about fifty percent the size of the Earth, Kepler-138b has a mass one-tenth of our planet. The exoplanet has a density that could very well be composed of rocks similar to Mars or Earth. Researchers, however, need further studies to come up with a more accurate finding of this rocky composition.
"A planet made of rock and water would be more stable against mass loss, and would imply that the planet formed at a greater distance from the star and migrated," study says.
The two larger Kepler exoplanets have measurement closer to the Earth's size. Kepler-138 could have a rocky composition, however Kepler-138d is unlikely to be as dense. Similarly, the three planets are least likely to produce liquid water on their surfaces as they all orbit close to their star.
The Kepler-138b was discovered through findings from NASA's spacecraft Kepler. Between 2009 and 2013, Kepler observed about 150,000 stars, eventually revealing a planet in regular transit in front of its star.
Science has always highly regarded determining the size of an Earth-size planet by measuring the amount of starlight the planet blocks several light years away. It also highly speaks of measuring a small planet's mass by measuring its gravity. The discovery of Kepler0138b's size and mass reveals how our astronomers today have done both, to an exoplanet half the size of the Earth.
"The detection of such a small exoplanet in a tight orbit could help to clarify how we fit into the big picture," Gregory Laughlin of UC Santa Cruz wrote in a commentary. Laughlin is not involved in the study.
