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Presented by Maggie Aderin-Pocock Astronomer & broadcaster

Academic consultant Professor Gilles Chabrier University of Exeter

Presented by Maggie Aderin-Pocock Astronomer & broadcaster

Academic consultant Professor Gilles Chabrier University of Exeter

Exoplanets - planets outside our Solar System - were first proposed in the 16th Century. Galileo realised our Sun was a star like any other and Copernicus showed how our Solar System's planets orbit around it. So it seemed fair to assume that planets existed elsewhere in the galaxy, orbiting stars of their own. But it would take hundreds of years to develop telescopes powerful enough to find out whether they did.

The first exoplanet was detected in 1995 and thousands more have been discovered since. At first, we could only see gas giants, similar to Jupiter. Now instruments are sensitive enough to detect planets tantalisingly similar to Earth. We don't know whether different planets could be home to alien life, very different to us. But for us, we'd need a planet similar to Earth to have any chance of living there.

But we may never find a suitable second home. Find out why.

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Where's the nearest planet where we could all live?

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Presented by Maggie Aderin-Pocock Astronomer & broadcaster Academic consultant Professor Gilles Chabrier University of Exeter Presented by Maggie Aderin-Pocock Astronomer & broadcaster Academic consultant Professor Gilles Chabrier University of Exeter Exoplanets - planets outside our Solar System - were first proposed in the 16th Century.

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Exoplanet Exploration: Planets Beyond our Solar System

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What is the habitable zone?

There's a helpful concept we use to help understand what distance from a given star you might expect to find planets with liquid water on their surface - liquid water being essential for life as we know it. It's called the habitable zone. Every star has a habitable zone, but where that zone lies is different for stars of different sizes and brightness. Credit: NASA/JPL-Caltech

When searching for possibly habitable exoplanets, it helps to start with worlds similar to our own. But what does "similar" mean? Many rocky planets have been detected in Earth's size-range: a point in favor of possible life. Based on what we've observed in our own solar system, large, gaseous worlds like Jupiter seem far less likely to offer habitable conditions. But most of these Earth-sized worlds have been detected orbiting red-dwarf stars; Earth-sized planets in wide orbits around Sun-like stars are much harder to detect.

And, of course, when talking about habitable exoplanets, we're really talking about their stars, the dominant force in any planetary system. Habitable zones potentially capable of hosting life-bearing planets are wider for hotter stars. Smaller, dimmer red dwarfs, the most common type in our Milky Way galaxy, have much tighter habitable zones as in the TRAPPIST-1 system. Planets in a red dwarf's comparatively narrow habitable zone, which is very close to the star, are exposed to extreme levels of X-ray and ultraviolet (UV) radiation, which can be up to hundreds of thousands of times more intense than what Earth receives from the Sun.

This infographic compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs. Credits: NASA, ESA and Z. Levy (STScI)

Where are we looking for life, and why?

Where are we looking for life, and why?

An old joke offers an answer: Asked why, on a dark night, he was looking for his missing car keys beneath a street lamp, the man answered, "because the light's better." Life on other planets might be like nothing on Earth - it could be life as we don't know it. But it makes sense, at least at first, to search for something more familiar. Life as we know it should be easier to find. And "the light's better" in the habitable zone, or the area around a star where planetary surface temperatures could allow the pooling of water.

Other similarities to Earth come into sharper focus in the search for life. Many rocky planets have been detected in Earth's size-range: a point in favor of possible life. Based on what we've observed in our own solar system, large, gaseous worlds like Jupiter seem far less likely to offer habitable conditions. But most of these Earth-sized worlds have been detected orbiting red-dwarf stars; Earth-sized planets in wide orbits around Sun-like stars are much harder to detect. Yet these red-dwarfs have a potentially deadly habit, especially in their younger years: Powerful flares tend to erupt with some frequency from their surfaces. These could sterilize closely orbiting planets where life had only begun to get a toehold. That's a strike against possible life.

Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates in the search for other potentially habitable worlds. G-type yellow stars like our Sun, however, are shorter-lived and less common in our galaxy.

The artist's conception shows a hypothetical planet with two moons orbiting in the habitable zone of a red dwarf star. More about stars ›

Stars slightly cooler and less luminous than our Sun - called orange dwarfs - are considered by some scientists as potentially better for advanced life. They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms. And, for every star like our Sun there are three times as many orange dwarfs in the Milky Way.

K dwarfs, are the true "Goldilocks stars," said Edward Guinan of Villanova University, Villanova, Pennsylvania. "K-dwarf stars are in the 'sweet spot,' with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life."

Exoplanet temperature, size, star type: the galaxy offers up a menu of worlds that echo aspects of our own, yet at the same time are vastly different.

Image credit: NASA

Next: Why we search for life

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national aeronautics and space administration

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The Habitable Zone | The Search For Life - Exoplanet Exploration: Planets Beyond our Solar System

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Have you heard of the habitable zone? You're in one right now. It's the distance from a star where you might find exoplanets with liquid surface water. We use the habitable zone to help focus our search for life beyond Earth.

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The Cold War between the United States and former Soviet Union gave birth to the space race and an unprecedented program of scientific exploration. The Soviets sent the first person into space on April 12, 1961. In response, President John F. Kennedy challenged our nation "to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to earth." It took eight years and three NASA programs -- Mercury, Gemini and Apollo - but the United States got to the moon.

Apollo 11 astronaut Buzz Aldrin on the Moon. Look for Neil Armstrong reflected in Aldrin's visor. During the following three-and-a-half years, 10 astronauts followed in their footsteps.
Image Credit: NASA

Project Mercury

Project Mercury, the first U.S. program to put humans in space, made 25 flights, six of which carried astronauts between 1961 and 1963. The objectives of the program were: to orbit a human spacecraft around Earth, to investigate a person's ability to function in space, and to recover both the astronaut and spacecraft safely. More than 2 million people from government agencies and the aerospace industry combined their skills, initiative and experience to make the project possible. Mercury showed that humans could function for periods up to 34 hours of weightless flight.

Mercury Astronauts
Image Credit: NASA
Mercury astronauts, the "Original Seven." On April 9, 1959, NASA introduced its first astronaut class. Front row, left to right: Wally Schirra, Deke Slayton, John Glenn, Scott Carpenter; back row, Alan Shepard, Gus Grissom, and Gordon Cooper.

The Gemini Program

The Gemini program primarily tested equipment and mission procedures and trained astronauts and ground crews for future Apollo missions to the Moon. The program's main goals were: to test an astronaut's ability to fly long duration flights (14 days); to understand how a spacecraft could rendezvous and dock with another vehicle in Earth orbit; to perfect re-entry landing methods; and to further understand the effects of longer spaceflights on astronauts. NASA selected "Gemini" because the word is Latin for "twins," and the Gemini was a capsule built for two.

Spacewalk
Image Credit: NASA
Gemini IV spacewalk, June 3, 1965. NASA astronaut Ed White became the first American to walk in space.

The Apollo Program

Exactly eight years, one month and 26 days after President Kennedy challenged Americans to reach for the Moon, Project Apollo landed the first humans on the lunar surface and returned them safely to Earth. The Apollo program also developed technology to meet other national interests in space, conducted scientific exploration of the Moon, and developed humanity's capability to work in the lunar environment.

The ascent stage of the Apollo 11 lunar module approaching the command module for docking before the crew returned to Earth. Image Credit: NASA

The Apollo program was hit by tragedy as the first crew prepared to fly. On Jan. 27, 1967, fire swept through the Apollo 1 command module during a preflight test on the Cape Kennedy launch pad. Astronauts Gus Grissom, Ed White, and Roger Chaffee lost their lives. NASA was not deterred, but rather changed how things were done to ensure the safety and success of future missions.
Image Credit: NASA Image Credit: NASA
Apollo 13
Image Credit: NASA
A makeshift arrangement of equipment, parts and duct tape on the Apollo 13 Lunar Module (LM) saved the crew's lives after an oxygen tank explosion in the Service Module left them with the LM to use as a "lifeboat." Using materials only found on the spacecraft, NASA engineers on the ground designed and tested a system that removed carbon dioxide from the LM; the Apollo 13 crew then made the system onboard, April 17, 1970, and returned safely to Earth.

Skylab

In 1973, Skylab expeditions paved the way for the International Space Station. The four, windmill-like solar arrays were attached to the Apollo Telescope Mount. Observations of the Sun were one of this space lab program's primary achievements.

Apollo Soyuz Test Project
Image Credit: NASA
In the 1970s, U.S.-Soviet political tensions that had accelerated the space race began to thaw. Competition gave way to cooperation between the two nations with the Apollo-Soyuz Test Project. International collaboration among many nations would become the norm during the space shuttle era and current cooperation in human spaceflight with the International Space Station. These partnerships have taught us more about the universe, improved our lives at home, and expanded the possibilities for future exploration into deep space.

Handshake
Image Credit: NASA
Astronaut Tom Stafford (foreground) and cosmonaut Alexei Leonov make their historic handshake in space on July 17, 1975. The Apollo-Soyuz Test Project docked together U.S. and Soviet spacecraft and paved the way toward international partnerships in space.

Space Shuttle Era

Over 30 years, NASA's space shuttle fleet-Columbia, Challenger, Discovery, Atlantis and Endeavour-flew 135 missions and carried 355 different people to space. Humanity's first reusable spacecraft, the space shuttle carried people into orbit repeatedly; launched, recovered and repaired satellites; conducted cutting-edge research; and built the largest structure in space, the International Space Station. The space shuttle pushed the bounds of discovery ever farther, requiring not only advanced technologies but also the tremendous efforts of thousands of civil servants and contractors throughout NASA's field centers and across the nation. Tragically, NASA lost two crews of seven in the 1986 Challenger accident and the 2003 Columbia accident.

Space Shuttle Columbia, the world's first reusable space vehicle, landing at NASA's Dryden Flight Research Center (now NASA's Armstrong Flight Research Center) at Edwards Air Force Base, California, April 14, 1981. Image Credit: NASA

Space Station Era

The International Space Station is a model for global cooperation and scientific advancements that is enabling growth of private industry in low-Earth orbit and development of new technologies to advance human space exploration. Built between 1998 and 2011, the space station has housed humans continuously since Nov. 2, 2000. Because molecules and cells behave differently in space, research in microgravity helps advance scientific knowledge. The space station is a U.S. National Laboratory, which the Center for the Advancement of Science in Space (CASIS) manages for research investigations that improve life on Earth. NASA has contracted with commercial companies SpaceX, Orbital ATK, and Sierra Nevada Corporation to deliver science investigations, cargo, and supplies to the crews living in space, and soon Boeing and SpaceX will transport astronauts to and from the station.

This picture of the International Space Station was photographed from the space shuttle Atlantis as the orbiting complex and the shuttle performed their relative separation in the early hours of July 19, 2011 Image Credit: NASA

Spot the Station
Image Credit: NASA
Nighttime view of the eastern U.S. and Canada from the International Space Station. You can see the station too. Go to Spot the Station and sign up for text and email updates of sighting opportunities.

National Aeronautics and Space Administration

NASA Official: Brian Dunbar

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