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.
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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?
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