What did the Buddha teach about aliens? What are the teachings of the Buddha about aliens or other planets that have lives? 

Is this the only place that can have life or human beings? 

Is it possible to be born a Buddha in some other universe or planet?
The Buddha talked about the "world Systems" and other humans in them. This is a fascinating topic for myself as someone interested in astronomy. Here are some links speaking about world systems and related topics.

The Dhamma is sort of like the laws of mathematics and physics that govern this universe, universal to all sentient beings, everywhere. This is why we spread our metta out into the universe. I usually go one step further and spread metta out to the multi-verse.

What Buddhists Believe
Venerable K. Sri Dhammananda Maha Thera: Other World Systems 

http://www.budsas.org/ebud/whatbudbeliev/300.htm

The Buddhist Concept of World
Bhikkhu T. Seelananda

http://bhavanasociety.org/resource/the_buddhist_concept_of_world/

The Buddha once explaining the world system said, ” Monks, as far as sun and moon revolve and illuminate all directions by their radiance, so far does the thousand-fold world system extend. And in that thousand-fold world system, there are thousand moons, a thousand suns, inhabited planets…. This thousand-fold world system is called culanikā-loka-dhātu or Minor World-System, which is the smallest unit in the universe (But all these change takes place, transformation takes place. When seeing this the instructed noble disciple is disenchanted, dispassion..)

I also find this sutta fascinating in which the Buddha appears to be speaking about a black hole..

http://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.046.than.html
Andhakara Sutta: Darkness 

"There is, monks, an inter-cosmic[1] void, an unrestrained darkness, a pitch-black darkness, where even the light of the sun &amp; moon — so mighty, so powerful — doesn't reach."