Since 1992, astronomers have found more than 4,000 planets around alien stars, but no one has seen water on a “habitable” world — until now.
The exoplanet K2–18b resides in the habitable zone around its star, where temperatures are just right for liquid water to pool on the surface of worlds. This distance, where temperatures are warm enough to melt ice, but not hot enough to boil away seas, is sometimes referred to as the Goldilocks Zone.
Now, an international team of astronomers have discovered water in the atmosphere of K2–18b. This marks the first time that water has been detected on an exoplanet orbiting in the habitable zone around its star.
“Finding water in a potentially habitable world other than Earth is incredibly exciting. K2–18b is not ‘Earth 2.0’ as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental question: Is the Earth unique?” Dr. Angelos Tsiaras of the UCL Centre for Space Exochemistry Data (CSED) states.
Warm Planet, Near a Star, Ample Radiation
This super-Earth, eight times as massive and 2.4 times the size of our home world, was discovered in 2015. K2–18b is one of hundreds of super-Earths, exoplanets larger than Earth but smaller than Neptune, known to inhabit alien planetary systems.
Astronomers found the world as it passed in front of its star as seen from Earth. The technique, known as the transit method, also allowed researchers to measure the size of the world. By measuring the gravitational tug the planet has on its parent star, it was possible to determine its mass.
The exoplanet orbits its star once every 33 days, at a distance from its sun just 14 percent of the size of the orbit of the Earth. Due to its close proximity to its star, the surface of the world is likely subject to greater concentrations of radiation than we experience on Earth.
With a density around 3.3 grams per cubic centimeter (compared to 5.5 on Earth), K2–18b may be a silicate world with a thick atmosphere, or a planet teeming with plentiful oceans.
The red dwarf star around which this surprisingly-inviting world orbits, K2–18, is found 110 light years from Earth. Discovered by the Kepler spacecraft in 2015, K2–18 has a mass 40 percent as large as our own sun, and a surface temperature of around 3,200 degrees Celsius (5,850 Fahrenheit).
Getting to Know the Neighbors
A number of questions must be answered when asking if an alien world might be considered friendly to alien life. Among these likely qualifications, a planet should be in the habitable zone around its star and it should have a solid surface and/or oceans of water or other liquids (such as hydrocarbons).
Many more worlds the size of Earth or slightly larger may soon be found, where moderate temperatures and atmospheric pressures allow bodies of water to accumulate.
“Super-Earths — planets lighter than ten Earth masses — around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations,” researchers report in the journal Nature Astronomy.
Data collected by the Hubble Space Telescope (HST) in 2016 and 2017 was studied, as researchers examined starlight as it passed through the atmosphere of the distant world. Investigators found the air on K2–18b is mostly hydrogen and helium, but it also contains water vapor. This atmosphere would quickly suffocate humans, making this world an uninviting place for our species, but perhaps not for life adapted to those conditions.
The search for exoplanets is due to get a significant boast from an array of new telescopes and instruments around the world, as well as in space. The TESS spacecraft alone is expected to find hundreds of super-Earths orbiting distant stars. The most advanced space telescope ever launched, The James Webb Space Telescope, is readying for a million-mile journey, and the European ARIEL mission will examine 1,000 exoplanets in great detail, beginning in 2028.
“He lost himself in the words and images conjured in his mind and for a while forgot… He found himself flying among stars and planets…”
― Carlos Ruiz Zafón, The Prince of Mist
“With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets,” stated Dr. Ingo Waldmann of UCL CSED.
Finding water on this temperate world takes us closer to understanding the composition and nature of worlds around other stars, providing us all a chance to know our stellar neighbors a little better.
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