He's saying the Earth, as we know it, is gone

Monat:

My name is Monat. [other prisoners gasp] Alice:
What is it? What are you? Monat:
I am not human, as you can see, but neither am I dangerous, I assure you. Jeff Hale:
[to the others]
It's all right. [turns back to Monat] Jeff Hale:
Incredible. Monat:
I am from a star system your astronomers named Tau Ceti. Jeff Hale:
Yes, Tau Ceti. Are there other extraterrestrials here? Monat:
Alas, I am the only one. Jeff Hale:
You died on Earth... You were on Earth. Monat:
You are 20th Century? Jeff Hale:
Yes, early 21st. I was an astronaut, just like you. Monat:
My ship charted a large asteroid approaching your planet. We hastened to warn your authorities, but your intentions were misread and we were shot down. I alone survived the crash. I tried my best under interrogation, but your scientists were skeptical of our advanced projections. Jeff Hale:
Chaos theory. Monat:
For your people, it was just that, a theory. For mine, it was a practice. As we predicted, the asteroid's course was deflected when it passed through the Jovian Belt. By then, it was too late to stop it. The impact would have equaled the detonation of 100 million of your hydrogen bombs. No life form could have survived. Alice:
Well, this is more preposterous than H.G. Wells. Monat:
Believe me. Ask around. Find someone who claims to have lived on Earth after June 18, 2039 AD. Alice:
What is it saying? Monat:
Use of masculine gender would be appropriate when addressing me. Jeff Hale:
He's saying the Earth, as we know it, is gone