The 'scientist' who discovered that planet
and now claims that "[sic] It's 100 percent
certain there is life on that planet" is
violating one of the most important tenets
of science: claiming 100 percent certainty
for a speculative opinion of his own.

In other words?

What he said was not science.

And if it was an opinion, as it appears, it
wasn't a very good one.
Sure, there can be life on that planet; there
might even be sentient creatures on that world.
But it's still speculation, with no credible hard evidence yet, if ever.

It can go either way: that planet could have just the right balance of gases, water, and atmospheric density to support life, or it could have a very dense atmosphere (theoretical models support this idea)
with plenty of CO2 and H20 vapor in it, turning that planet into an extra-large Venus.
Even in a habitablezone around a star,
a large planet can have a runaway greenhouse effect.

But you know what? Even if earth is the only planet
in all creation that has life, I wouldn't despair
or become melancholy: for I see in the human species special abilities, talents, understanding and form
that are remarkable and purposeful.