Could there be life on Kepler 22-b?
Will we ever find an exoplanet just like Earth? With oceans, trees, animals, and maybe...some
kind of humanoid aliens? How will this planet be? Maybe
it will be smaller than Earth, maybe bigger. Astronomers are now looking for these kinds of
planets, but it is not so easy to spot them. With the help of our current techniques, we are
able to spot a lot of massive and large planets, but it's way harder to detect Earth-sized ones.
Of course, there are some exceptions. The first planet to ever be found
similar to Earth was Kepler 22-b. It was detected in 2009 and today
we know this amazing planet is orbiting in the habitable zone of its host star.
But what's a habitable zone? And what are the main
features of Kepler 22-b? There could be life on this planet?
Let's find out!
First of all, how do astronomers find an exoplanet?
There are actually various techniques, but the most used are direct imaging,
radial velocity, and transit method. As of today, we know that different
techniques are more sensitive to different kinds of planets.
For example, the direct imaging technique makes use of large telescopes fitted
with adaptive optics, which boosts their performances, to directly image an exoplanet.
This technique is most sensitive to the warmer, younger, and more massive exoplanets,
that have very stretched orbits. In fact, the more stretched an orbit is,
the bigger its angular size in the sky, and the easier it is to spot it!
The radial velocity technique makes use of the well-known Doppler effect. Astronomers
basically employ a spectrograph to get the host star spectrum. Since the star and the planet
are bound to each other, they are both going to move around their common center of mass.
This means that, during an orbit, the star will move back and forth. This
kind of harmonic movement will show in the spectral lines' shift, - the Doppler shift.
The technique is most sensitive to exoplanets with a large mass orbiting close to their
host star perpendicular to the plane of the sky, because these massive planets attract
the star more, causing the amplitude of the harmonic oscillation to be bigger.
The last technique, one of the most used and efficient, is the transit method.
This is actually a game of lights and shadows. The exoplanet is detected by measuring a periodic
decrease in the flux received on Earth from the host star. In fact, imagine having a
star and looking at it. At a given time, a planet happens to cross the stellar disk.
What you would see is a sort of “small eclipse” of the stellar disk, where the luminosity of the
star decreases as a consequence of the exoplanet transiting in front of it.
This technique is most sensitive to large exoplanets orbiting close to their host star.
In the case of our planet, Kepler 22-b, the transit technique was used.
The telescope employed for this kind of study is the famous Kepler space telescope,
a powerful instrument for searching for exoplanets.
Kepler discovers planets and planet candidates by measuring dips in the
brightness of more than 150,000 stars to search for planets that cross in front,
or "transit," the stars. It requires at least three transits to verify a signal as a planet.
What makes Kepler 22-b so special? Well, it was the first planet confirmed to
be in the so-called “habitable zone” of a star. This is defined as the orbital region around
a star in which an Earth-like planet can possess liquid water on its surface and possibly support
life. Liquid water is essential to all life on Earth, and so the definition of a habitable zone
is based on the hypothesis that extraterrestrial life would share this requirement.
Of course, for a planet to host life as we know it, it is not sufficient to only have liquid
water on the surface, and further studies are needed, but at least...it's something!
So let's see what we know about this possible Earth's twin.
We know it is located 600 light-years away. While the planet is larger than Earth, its orbit of 290
days around a sun-like star resembles that of our beautiful world. The planet's host star belongs
to the same class as our sun, called G-type, although it is slightly smaller and cooler.
The brightness Kepler 22-b gets from its star is about 25% less than the one we get from the Sun.
We also know that Kepler-22b is the smallest planet yet found to orbit in the middle of
the habitable zone of a star similar to our sun. Just to give some numbers,
its radius is 2.4 times the radius of Earth.
Scientists don't yet know if Kepler-22b has a predominantly rocky, gaseous,
or liquid composition, but its discovery is a step closer to finding Earth-like planets.
Finding planets of the size of Earth or smaller is no picnic!
In fact, as we said before, most of the current and modern techniques allow us to find massive
planets, large in size, and with highly eccentric orbits. But from time to time,
scientists happen to be lucky. For example, other small planets orbiting stars smaller
and cooler than our sun recently were confirmed on the very edges
of the habitable zone, with orbits more closely resembling those of Venus and Mars.
"Fortune smiled upon us with the detection of this planet," said William Borucki,
Kepler principal investigator at NASA Ames Research Center at Moffett Field,
Calif., who led the team that discovered Kepler-22b. "The first transit was captured
just three days after we declared the spacecraft operationally ready.
We witnessed the defining third transit over the 2010 holiday season.
Just to understand what we are talking about, take a look at this plot.
On the x axis, we have the semi-major axis
of the planet, which means, the distance of the planet from the host star. This increases as we
move from the origin to the right. On the y axis, we have the planet's mass. It increases upwards.
The shapes represent the various detection techniques. As you can see,
the transit method and the radial velocity method are the ones that,
as of today, can help us find the less massive planets, if they are very close
to their host star. The black triangle, indicating the so-called timing technique,
seems to be promising, but we will need more data from teams using this technique.
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The NASA site on the Kepler mission estimates
the mass of this planet to be 36 to 124 times the mass of the Earth.
Considering the range of possible masses between 6.36 M ⊕ (Earth
masses) and 124 M ⊕ (but in these cases, the most probable mass is always the smallest),
it is also easy to obtain the interval of the possible surface gravity, which
goes from the most probable value of 1.11 g to the exorbitant and improbable value of 21.5 g.
However, if we use the only data we have if the planet was composed of the
same elements that make up the Earth, distributed in the same proportions,
then it would have a mass of 13.8 M ⊕ and a surface gravity of 2.4 g.
But we know this planet's diameter is 2.4 times that of the Earth.
This might tell us Kepler 22-b has a different composition from that of Earth.
Some astrophysicists say it could be an ocean planet, but what do they exactly mean with that?
An ocean planet would be a planet consisting of a rocky core,
surrounded by an extensive mantle of ice and water, on which a vast surface ocean extends.
Can you imagine? If you were to live on Kepler 22-b, you would have to swim a lot! However,
it is also possible that it has a composition rich in volatile elements
in its outer envelope, and this would make the planet the smallest known gaseous dwarf.
Kepler 22-b is really close to its host star, much more than Earth is to the Sun, but the brightness
of the star is lower than the solar one. If we combine these two parameters together, we can say
the planet has a moderate surface temperature, which would be ideal for life as we know it.
Of course, everything depends on the atmosphere. In its absence, it would be really cold,
something like -11°C. But if an atmosphere existed, and if the planet had a greenhouse
effect similar to Earth's, the planet would have a nice average temperature of 22°C.
The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope
to review observations on planet candidates the spacecraft finds.
The star field that Kepler observes in the constellations Cygnus and Lyra can only be
seen from ground-based observatories in spring through early fall. The data
from these other observations help determine which candidates can be validated as planets.
From what we know, Kepler 22-b is not the only planet found in the habitable zone so far.
Kepler also has discovered more than 1,000 new confirmed exoplanets,
nearly doubling its previously known count. A bunch of these candidates
are near-Earth-size and orbit in the habitable zone of their host star.
Candidates require follow-up observations to verify they are actual planets.
Of the 1,780 confirmed planets beyond our solar system, as many as 16 are located in
their star’s habitable zone, where conditions are neither too hot nor too cold to support life.
For instance, if planets orbit too close to their respective suns, surface water
evaporates into space. The planet is too hot, and the conditions for a habitable zone are not met.
On the other side, a planet might be too cold. This is the case for planets
orbiting far from their host star. In this zone, any surface water remains frozen.
Size also matters: A planet that’s too small can’t maintain an atmosphere; one that’s too
large will have a crushing atmosphere. A recently detected planet 493 light-years from Earth,
Kepler-186f, is close to Earth's size and is located in its solar system's habitable zone.
As of today, most confirmed exoplanets are much larger and hotter than Earth, characteristics
that make them relatively easy to detect, but not interesting for the searching for life.
Unfortunately, most of the planets found in the habitable zone turned out to be flops,
in the sense that follow-up studies showed they are not able to host life.
For instance, one of the most promising planets
was Gliese 582 c, the first super-earth found in the habitable zone. However,
this planet was later found to have extreme surface conditions, that may resemble Venus.
This video ends here! What do you think about Kepler 22-b?
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