In January 2015, NASA announced three confirmed planets that are nearly Earth’s size that orbit within their suns’ habitable zones, where liquid surface water might exist. One of the new planets, Kepler-438b (KOI-3284.01, unofficially dubbed Phaeton) is the most Earth-like planet outside our solar system in terms of its radius, density, escape velocity and surface temperature. However, researchers soon discovered that its sun has superflares every few hundred days, presumably stripping the planet of its atmosphere and exposing it to radiation that renders it uninhabitable. Kepler-438b is also 473 light years from Earth, making surface exploration impossible.
The quest for inhabitable, Earth-like worlds continues closer to home, where researchers are exploring a few planets and moons in our own solar system.
Ever since 19th-century astronomer Giovanni Schiaparelli speculated that Mars’ canals might spread water and life over its surface, the Red Planet has been seen as a candidate for hosting extraterrestrial life. The next planet out from Earth, a day on Mars lasts about 24 hours and 39 minutes and it has a tilt similar to Earth’s that creates seasonal variations.
Mars and Earth share an approximately equal amount of surface dry land because Mars currently lacks an ocean. Mars displays geologic processes and surface features similar to Earth, including volcanoes, mountains and canyons. The Martian atmosphere is currently dominated by carbon dioxide, but NASA scientists believe that Mars once had an atmosphere and possibly oceans that have been stripped away by solar winds. The announcement followed a September NASA report confirming evidence of briny liquid water flowing on Mars.
Scientists are figuring out how to continue the search for life on Mars without contaminating the planet with Earthly microbes, which can be carried by robots. NASA hopes to resolve this dilemma in time to send humans to Mars by 2030.
Continuing past Mars to Jupiter, the next place of interest is Europa, Jupiter’s sixth moon. Galileo discovered Europa by telescope in 1610. Five centuries later, in 2011, NASA began considering a proposed mission to Europa’s oceans to look for life.
Beneath a thin atmosphere of oxygen, Europa’s surface is crusted with a layer of water ice. Scientists believe the ice is approximately 62 miles thick and covers a saltwater ocean containing more water than Earth’s oceans. Cracks in Europa’s ice suggest underlying tidal and tectonic activity that warm the moon’s ocean. University of Arizona professor of planetary sciences Richard Greenberg has argued that Europa’s oceans contain enough oxygen to support microorganisms and macrofauna.
Scientists are currently studying icy waters on Earth, such as Antarctica’s Lake Vostok, to better understand conditions on Europa.
Scientists who want to explore Europa have also proposed a mission to Enceladus, Saturn’s sixth-largest moon. Like Europa, Enceladus has an icy surface believed to cover a watery ocean. Volcanoes near Enceladus’ poles spout plumes into the atmosphere, helping create Saturn’s second outermost ring. The plumes contain salty water vapor, one indicator of a salty subsurface ocean. Scientists analyzing Enceladus’ plumes believe they may be similar to those active in the Atlantic Ocean’s Lost City hydrothermal field, which is home to a variety of microorganisms that can thrive under such conditions.
Titan, Saturn’s largest moon, is considered to be one of the most Earth-like objects in the solar system. Like the Earth, Titan has a thick atmosphere blanketing a rocky surface. Interactions between Titan’s atmosphere and the sun’s magnetic field and radiation generate a wind of hydrocarbons and nitriles that blow from the moon’s polar regions into space, a phenomenon similar to winds from Earth’s poles.
The Cassini spacecraft mission revealed rivers and lakes of liquid ethane and methane on Titan’s surface, and scientists suspect a subsurface ocean similar to that beneath the ice of Europa and Enceladus. Astronomers believe Titan resembles a frozen version of Earth that shows what our planet looked like before life started pumping oxygen into the atmosphere.