Modern science has shown us how vast the known universe is, with about 1,000 named planets seen with high powered telescopes in space. Other solar systems have been seen, including those in varying stages. To say that we are alone in this universe as the only planet with life on it would be very foolish considering the scientific evidence of just how vast the universe is with so many stars (suns) and their own planets revolving around them.
The Buddhist cosmology talks of 10,000 world systems to say that there are innumerable solar systems and planets. Each world system has its own 31 planes of existence (at least to number 16, see below). The Buddha stated that "the infinite world spheres are incalculable" (KN, Buddhavamsa 1.64).
The Buddhist cosmology has even taken into account the possibility of our solar system expiring or being destroyed. If a being is in one of the first 16 planes and the world system (solar system) is destroyed for whatever reason, then that being must be reborn into a plane of existence at number 17 or higher. If this being does not get reborn to plane 17 or higher, then that being is reborn in a different world system. That different world system would be an alien solar system many light years from us.
We know from science that planets and solar systems do go through various stages and do terminate at one point, such as when the sun uses up and finishes its energy. We also know that solar systems reconstitute and re-form and re-evolve after many billions of years. This is all compatible and almost word for word how the Buddha described it:
“He recalls to mind his various temporary states in days gone by – one birth, or two or three or four or five births, 10 or 20, 30 or 50, a 100 or a 1,000 or a 100,000 births, through many cycles of cosmic contraction and cosmic expansion . . . Now there comes a time, when sooner or later, after the lapse of a long, long period of contraction, this world-system passes away. And when this happens beings have mostly been re-born in the World of Radiance, and there they dwell made of mind, feeding on joy, radiating light from themselves, traversing the air, dwelling in glory; and thus they remain for a long, long period of time. Now there comes also a time, friends, when sooner or later, this universe begins to re-evolve by expansion.” (Brahmajala Sutta, Digha Nikaya)
According to the Buddha the universe has no ultimate beginning or end (Samyutta Nikaya 2. 178), but rather forms, dissolves and reforms again over a vast period of time (D.III,84).
The Buddha also says that within these world systems are "thousands of suns, thousands of moons, thousands of continents" (Anguttara Nikaya 1. 227). He also speaks of the "the black, gloomy regions of darkness between the world systems, where the light of our moon and sun, powerful and majestic though they are, cannot reach" (Majjhima Nikaya 3. 124), suggesting that these galaxies are great distances away from each other in space.
In 2013 it was reported that "Many more planets are thus out there, zipping undetected around their parent stars. Indeed, a team of researchers estimated last year that every Milky Way star hosts, on average, 1.6 worlds — meaning that our galaxy perhaps harbors 160 billion planets." 
The likelihood that we are alone in this universe is decreasing to near nil by the day. Which means that there must be other "Buddhas" in these other worlds; no conflict to Buddha-Dhamma, which never stated that the Buddha of our time is the only one.