The scientists are looking for exoplanets, but despite all their efforts, they couldn't
find any so far.
Isn't it very extraordinary that the universe is so big, but our planet seems to be the
only one with conditions right for life and that it looks very specially created and very
well protected?
Prior to 1995, there had been many attempts for detection of planets around other stars,
and these would be extrasolar planets or exoplanets.
And every attempt or every discovery was proven to be either subject to doubt or just incorrect.
But in 1995, two teams discovered planets, a planet around 51 Pegasi.
This was a real observation of a planet around another star.
At that point, there was a very great sense of euphoria in the scientific community because
for the first time, we had begun to have techniques that are sensitive to seeing planets around
other stars.
So in 1995, the expectation was that there are planets abundantly around other stars and
many planets like the Earth will be very easily observable.
If you look at statements made, for example, to national academies or to the Congress of the
United States by American scientists, they talk about billions of Earths just within our galaxy.
But in fact, since 1995, as our techniques have even more improved, yes, what we have seen is thousands
of planets.
We have seen thousands of planets around other stars.
But then when you ask to compare those planets with the Earth, what you find is that planets,
yes, there are, but Earth-like planets, there essentially are not because the requirements for
an Earth-like planet are many, many requirements.
The planet itself has to be situated in what is called the habitable zone.
A habitable zone is essentially a distance from the sun where the temperature of the, the
mean temperature of the planet is one in which liquid water can exist at least somewhere on
the planet.
So if you're too close, what you end up having is that there is excess heat and the heat evaporates
the water which causes a greenhouse effect.
This causes an effect that planetary geologists call the moist greenhouse that then leads to
a runaway greenhouse and so that all the water essentially evaporates.
When it evaporates, it gets to the upper reaches of the atmosphere where a particle process called
sputtering essentially sends particles from the sun and knock off the hydrogen off the upper
atmosphere of the planet and the planet becomes desiccated.
So it doesn't just stay moist, it just actually gets desiccated.
On the other hand, if the planet is too far from the sun, the star, then the moisture will
snow down on the, onto the surface of the planet where the albedo, which is the reflectivity
of the planet, goes up.
As it does that, the planet temperature drops which causes more snowing and you end up with
a situation called the snowball Earth.
So you have a runaway greenhouse on one end and a snowball on the other end and only in
the middle is a perfect state.
Now calculations have been done to say from the Earth, how far closer can I go before I
get into a runaway greenhouse?
Some of these calculations put it at 1%.
We're 1% closer and we get into a runaway greenhouse.
So we are very, very perfectly situated.
And that's just from the standpoint of distance.
Then the point is that not all stars are the same.
We have a very stable, energetic star, a massive, energetic, stable star.
If our star was one of the more common stars, like an M star, which would be a cooler star,
the habitable zone is closer.
I have to be closer to be habitable, to have the moisture.
But when I'm that close, it turns out I'm very close to a star which is very unstable.
M stars have a lot of strong energy output changes that also damage the planet.
The other thing that happens is that the planet's day and night cycle, which is governed by its
spin, as you get closer there's a gravitational effect called tidal friction and tidal locking,
which essentially locks the planet so that one side of the planet is the only site that's
seen.
So one side of the planet totally burns up, the other side just, you know, gets just super
chilled.
These are just two of a great number of conditions that have come together for the Earth.