Just about 30 years ago, we didn’t have
any scientific proof of a planet outside our solar system. Today, thanks to advances in technology and
scientific research, we've discovered over 5,300 exoplanets - alien worlds that are out
there waiting to be explored. At first, these discoveries mostly involved
large gas giants, but as our capabilities have expanded, we've started to uncover smaller,
Earth-like exoplanets. Scientists have already found more than 50
exoplanets with masses similar to Earth, and over 800 worlds with radius less than one
half times that of Earth. We don’t know of many such planets orbiting
in the habitable zone of their parent stars - where the conditions are just right to support
life. But that's already starting to change. As our search for habitable planets continues,
we're beginning to discover entire planetary systems with more than one potentially habitable
world. And we already know about at least one such
system in our cosmic neighborhood. [LOGO] Our Milky Way galaxy has numerous compact
systems that are centered around stars similar to our Sun. However, planets orbiting closely to these
stars are usually too hot and therefore unsuitable for supporting life. But when it comes to colder, fainter stars,
the habitable zone around such stars is much closer. This is Gliese 667 [GJ 667], a triple star
system located in the constellation of Scorpius just about 23 light-years from Earth. It contains the first known example of a system
where a low-mass star has multiple potentially habitable rocky planets orbiting within its
Goldilocks Zone. Similarly to Alpha Centauri, the system has
three stars. GJ 667 A is a K-type main-sequence star and
the largest one in the system. This orange-red dwarf has a mass of 73%, a
radius of 76%, and a visual luminosity only about 12% that of the Sun. At an average distance of 12.5 AU is its companion,
GJ 667 B. It’s also a K-type orange-red dwarf about 69% the mass of the Sun, and it
radiates just about 5% of our star’s visual luminosity. And just like with Alpha Centauri, the most
interesting is the third star. Gliese 667 C is an M-type red dwarf with a
mass and radius only a third of the Sun [Mass: 33% x Sun; Radius: 34% x Sun]. It's also incredibly faint [Luminosity: 1.4%
x Sun] and a comparatively cold star with a surface temperature of 3,775 Kelvin [Sun
surface temperature: 5,772 Kelvin]. Yet, despite its small size, GJ 667 C has
a surprisingly rich planetary system. Initially, scientists thought there were only
three exoplanets orbiting Gliese 667 C, but after revisiting existing data and making
additional observations, they discovered there may actually be six planets in the planetary
system, with three or even four of them being potentially habitable super-Earths. So what exactly is a "super-Earth"? The term is used to describe a planet that's
larger than Earth, but not quite as large as the gas giants like Jupiter and Saturn. These planets can be made up of rock, or a
mixture of rock and ice, and they may have atmospheres that could support different life
forms. The closest planet to the star, Gliese 667
Cb, is a scorching hot world at 200°C [392 °F]. Being the most massive one in the system,
about 5.5 times that of Earth, this exoplanet probably has a very thick atmosphere and orbits
its host star in just 7 days. The three potentially habitable planets in
the Gliese 667 system are all located further away from their host star, and they all have
masses between one and five times that of Earth, making them great candidates for habitability. Gliese 667 Cc, the next closest planet, orbits
in the inner edge of the star’s habitable zone. It has a mass about 3.8 and a radius 1.8 times
that of Earth, and its year lasts just 28 Earth days. With the Earth Similarity Index of 0.85, it
is known as the “holy grail” of extrasolar planets. Because of the low energy output, the habitable
zone around the red dwarf GJ 667 C is located very near to the star, ranging from 0.11 AU
to 0.23 AU, and is entirely contained within the orbit of Mercury. To compare, Earth is located at about 1 AU
from the Sun. Our planet would be an ice world if it orbited
star C at that distance. GJ 667 Cc orbits its parent star eight times
closer, at approximately 0.12 AU, revolving around the red dwarf every 28 days. Because of such proximity, the exoplanet is
likely to be tidally locked to the star, with eternal day on one side, and eternal night
on the other. Drastic temperature differences between the
two sides probably have a major impact on the global climate on the exoplanet. Gliese 667 Cc gets about 10% less light than
Earth does from the Sun. But since most of the light it receives is
infrared, the planet gets about the same amount of energy as our planet does from the Sun,
which would help retain water on its surface and result in a similar climate to Earth. Because scientists don’t know for sure if
the planet has an atmosphere, and how thick it could be, it’s impossible to predict
the exact surface temperature on Gliese 667 Cc. If the planet does have an Earth-like atmosphere,
it would transfer heat and equalize temperatures across the entire planet with a pleasant 30°C
[86°F] on the night side. Living on such a planet would be a much different
experience. GJ 667 Cc receives a faint reddish light from
its star. The other two stars, Gliese 667 A and B, are
located at a distance of about 230 AU - much further than the distance between Pluto and
the Sun, and outside the planetary system. However, the two other suns would still be
seen as a pair of bright stars visible in the daytime, and at night, they would shine
as bright as the full moon. And our Sun would appear as a distant star. Unfortunately, the nearby red dwarf is known
to emit flares or intense bursts of radiation, and energetic particles up to a thousand times
stronger than flares on our Sun. This could be problematic for any potential
life on the surface of Gliese 667 Cc as the planet is located close to its flaring host
star. And the strong magnetism of the red dwarf
may cause starspots that can reduce the energy output of the star by up to 40% for months,
which, combined with the lack of ultraviolet light emissions, would be another issue for
the formation of life as we know it. Living on Gliese 667 Cc would be nothing like
what we’re used to also because of how massive it is. The higher mass of the exoplanet means different
gravitational acceleration on its surface. This world is rocky, and so the gravitational
acceleration would be up to 60% higher than what we experience on Earth. A person weighing 75 kg [165 lbs] on Earth
would weigh as much as 120 kg [265 lbs] on Gliese 667 Cc. In addition, a planet with higher mass can
hold a more massive atmosphere, leading to higher atmospheric pressure at the planet's
surface. In case it has an atmosphere similar to Earth's,
the atmospheric pressure would only be a few times higher, but if the exoplanet has a Venus-like
atmosphere, the pressure could be several hundred times greater, equivalent to the water
pressure several kilometers deep in Earth's oceans. Despite its location in a habitable zone,
Gliese 667 Cc may not have the same conditions as Earth. Life forms on Gliese 667 Cc may have to adapt
to fluctuating and low light, a potentially high atmospheric pressure, and frequent flares. But this doesn’t mean life cannot form on
such a world. We’ve already seen examples of remarkable
life adaptability on Earth. The two other potentially habitable planets
are almost identical. Gliese 667 Ce [Mass: 2.7 Earths, radius: 1.45
x Earth] and Gliese 667 Cf [Mass: 2.7 Earths, radius: 1.45 x Earth] are both located farther
out from their parent star, meaning they receive less energy. This could make them too cold to support life
as we know it. But, unlike with GJ 667 Cc, having a thick
atmosphere would be beneficial for potential life on these planets as it would trap heat
and maintain favorable temperature conditions. Spotting three such worlds in the habitable
zone of the same planetary system is extremely rare, but four is almost unthinkable. According to one study, five planets in the
Gliese 667 C system are estimated to receive solar radiation ranging from 20-200% of the
current exposure of Earth to the Sun, which makes them all candidates for potential habitability. But there are other factors in play. Scientists have determined that for a planet
with a mass equal to Earth, the habitable zone around Gliese 667 C has two boundaries. The inner boundary lies between 0.095 and
0.126 astronomical units from the star, while the outer boundary lies between 0.241 and
0.251 AU. Any planet orbiting within these distances
from star C may be able to sustain life as it would have the right conditions for liquid
water to exist on its surface. If a planet is too close to its star, the
heat will cause water to turn into vapor and escape, making the planet uninhabitable. This happens because water vapor is a greenhouse
gas, which can trap heat and cause temperatures to rise to intolerable levels. Only the planets at the inner edge of the
habitable zone with a larger mass are more resistant to the moist greenhouse effect. On the other hand, if a planet is too far
away from its star, like the outermost Gliese 667 g, it risks being covered in ice. While gasses like CO2 can warm the planet
and prevent this, too much CO2 can actually cool the planet down by reflecting light away. So there’s a limit to how much CO2 can help
warm a planet. In 2013, astronomers made an announcement
that Gliese 667 C has a minimum of six planets, and there's a possibility of a seventh planet,
designated as GJ 667 Ch. Although highly controversial, the exoplanet
could be the smallest one found so far around Star C, with a mass of at least 1.1 times
that of Earth, located just between the planets C and B. Because of its mass and proximity
to the parent star [0.0893 AU], the planet H would be too hot for any life to form. But planets F and E are confirmed to orbit
in the habitable zone. And although the estimated location of the
planet D is outside the outer edge of the habitable zone, its orbit is still uncertain. This means Gliese 667 Cc, Cf, Ce, and possibly
even Cd are all potentially habitable worlds. The discovery of densely packed planetary
systems around M-dwarf stars, such as Gliese 667 C, points to the existence of numerous
populations of planetary systems out there, each with several potentially habitable planets. And since M dwarfs account for more than 70%
of all stars in our cosmic neighborhood, the number of such promising planetary systems
in our galaxy is likely to be much greater than ever thought. Instead of searching for a single potentially
habitable planet among ten stars, scientists can now focus on one star to find multiple
Earth 2.0 candidates. As new and advanced telescopes are being developed,
our ability to uncover the mysteries of the universe is growing at an exponential rate. What star system would you like to hear about
next? Sound off in the comments. Stay tuned here to be the first to hear about
the next big discovery in the cosmos, and make sure you click the super thank you button
if you enjoyed the video. Thanks for watching!
