Although we still can't travel to Mars as easily as commuting to work, the very idea of being able to go on a trip to another planet and live there someday has long piqued the interest of scientists and outer space fans alike.
A group of scientists in the future far, far away may be able to design and develop a real-life Millennium Falcon with a hyperdrive that could take us beyond the limits of our solar system - one that would probably cost us a hundred years of research and more than trillions of dollars.
Engines with hyperdrives only exist in the realm of science fiction. Nevertheless, that doesn't stop humans from creating hypotheses or theories.
Just like in sci-fi books or movies, the human race would have to travel in space and vacate the Earth for pure conquest because it has finally managed to successfully break the bounds of science.
The reason, however, could be far worse. In the far away future, the world might fall ill to climate change or perhaps be the site of a zombie apocalypse.
Nonetheless, astrophysicists from different parts of the world have been searching for planets where humankind could possibly survive. Over the years, the list of potentially habitable planets has increased.
Potentially Livable Planets
NASA's Kepler has already discovered at least 1,000 planets since 2009, 12 of which are less than twice the size of our own planet. These 12 planets orbit their host stars at the right distance to make them neither too cold nor too hot to potentially contain water.
The official authority to list down habitable exoplanets is the Planetary Habitability Laboratory. The agency's Habitable Exoplanets Catalog contains 29 exoplanets that are considered to be livable, and the list will be updated soon because of the discovery of a new exoplanet.
1. Wolf 1061c
In December last year, astronomers in Australia discovered a possibly habitable super-earth called Wolf 1061c, a planet 14 million light years away from Earth, located in what scientists call the "Goldilocks" zone. In this zone, temperatures are just right for water to exist.
"It is fascinating to look out at the vastness of space and think a star so very close to us - a near neighbour - could host a habitable planet," said Dr. Duncan Wright, the lead author of the research. The exoplanet Wolf 1061c will soon be added into the catalog.
On July 23, NASA confirmed the discovery of Kepler-452b, an exoplanet nearly the size of the Earth that is found in the Goldilocks zone.
This exoplanet orbits a Sun-like host star. Its diameter is 60 percent larger than that of Earth, and scientists theorize that this exoplanet is likely to be rocky.
Scientists also found that this exoplanet orbits around its star for 385 days. Hence, calling it the bigger and older cousin of Earth.
The exoplanet Kepler-186f is about 500 million light-years away from our planet, and is 10 percent bigger than it. Scientists also believe that this exoplanet is mostly rocky.
Like Kepler-452b, Kepler-186f is another one of Earth's distant cousins. "Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth," said scientist Thomas Barclay.
Orbiting the habitable zone of its host star is Kepler-22b. This exoplanet is about 600 million light years away from Earth, and has a radius that is 2.4 times greater than our planet's.
Scientists noted that Kepler-22b's planetary temperature is almost the same as the Earth's, and that its surface temperature is a life-friendly 72 degrees Fahrenheit.
5. Gliese 581d
In 2009, scientists discovered that the exoplanet Gliese 581d had the potential to support water on its rocky surface.
The super-earth's mass is about 7 times greater than that of our planet, and it would be two times bigger in mass. If possible, the exoplanet could contain an atmosphere and a liquid ocean.
Below is the official list of habitable planets from PHL.
(Photo : Planetary Habitability Laboratory)
How Do Scientists Determine Which Planets Can Host Life?
Because stars and planets beyond our solar system are too distant and unreachable, a team of scientists created a technique that could determine which exoplanets have the potential to accommodate human life. For them, finding out which planets are livable is highly dependent on the properties of the host star.
"If you don't know the star, you don't know the planet," said Jaymie Matthews, Astrophysics Professor at the University of British-Columbia and co-author of the study featured in the journal Science Advances.
Matthews and his colleagues took advantage of the stars' gravitational pull and looked into the differences in each star's brightness levels.
Why do brightness levels in stars vary? Imagine a pot of boiling soup. Its surface is hot and unstable, and if you hover your hand on top of the pot, you will feel the heat from the boiling soup rise onto the palm of your hand.
This happens because of convection--where the heated air or liquid is transferred away from the source, carrying energy and surface turbulence.
"Substitute the chicken broth held in a pot by Earth's gravity with hydrogen gas held together by a star's gravity," said Thomas Kallinger, Matthews' co-author. "Convection in the gas can trigger vibrations in the star."
'The Boy Who Cried Earth'
Gravity is extremely crucial to this study as it is to Earth. It will aid scientists in finding the properties of host stars, their radii and masses, as well as in identifying the specific properties that make surrounding exoplanets habitable.
For instance, to know the size of the exoplanet, one must know the size of its host star. Matthews said that not knowing the properties of the host star may bring confusion.
He explained that scientists may possibly find a planet around a Sun-like star, but they might mistake it for a livable Earth-sized world even when its star is actually a giant.
"Remember the fable 'The Boy Who Cried Wolf?' No astronomer wants to be The Boy Who Cried Earth and later has to say 'Never mind. My bad,'" said Matthews.
Matthews and his team based their new technique on data provided by Canada's Microvariability and Oscillations of Stars (MOST) telescope and Kepler satellites.
Matthews said whichever the distance of the first habitable planet is, it will be a footnote in the history books. "The headline will be 'We found life!'"