Ask A Scientist

How is it that our earth is the only one with life on it?

Asked by: Meghan Swartz
School: Maine-Endwell Middle School
Grade: 6
Teacher: Mr. Wagstaff
Hobbies/Interests: Sports (soccer, softball, basketball), hanging out with friends.
Career Interest: Becoming a famous soccer player, then owning a restaurant called Happy Days

Answer from Hiroki Sayama

Director, Collective Dynamics of Complex Systems Research Group, Binghamton University

Research area: Complex systems, artificial life, mathematical biology, computer and information sciences
PhD school: University of Tokyo
Interests/hobbies: Traveling, walking, swimming
Family: Wife, Mari, two sons, Takehiro (13), Yukihiro (8)
Web page address:  

You think so? Honestly, I think not. I am almost certain that there are other planets, probably tons of them, in this Universe where living things proliferate. Many prominent scientists, such as Stephen Hawking and the late Carl Sagan, agreed with this view, so it's not just my crazy idea. The Reason? Because the Universe is just too big for us to be the only life.

Here is a quick, ballpark estimate of how likely extraterrestrial life would be. It is known that there are at least one hundred billion stars in our Milky Way galaxy, and there are at least one hundred billion similar galaxies in the observable Universe. So, if you multiply those two large numbers, we learn that there are at least ten thousand quintillion (=billion times billion) stars in the Universe.

There is currently some debate on how likely it is for those stars to have Earth-like planets, conditioned at the right temperature and furnished with the right materials such as water, carbon and other matters that are necessary for life to emerge. Some say only one in several hundred of those stars could have Earth-like planets, while others are much more optimistic, saying as many as 25% of those stars would do. Recent discoveries are indeed suggesting that many of those stars actually have Earth-like planets around them. But here, let's be super-conservative to assume that only one in a million of those stars could have a habitable, Earth-like planet. This brings the number of Earth-like planets in the University down to ten quadrillion (= million times billion). Much smaller, but still a huge number.

The next question is how likely life will form spontaneously on such habitable planet (yes, life can emerge spontaneously from non-living substances through chemical evolution - this is actually my favorite research area, but it is a long story, so let's leave it out for now). Scientist generally agree that the formation of life on a habitable planet is probably much more likely than people might think. The history of life on our Earth is a positive example; there are evidences that the earliest forms of life on Earth existed more than 3.5 billion years ago, which was just one billion years after the formation of the planet. This implies that life can emerge "as soon as" there are right conditions for them (in a very long time scale used in geosciences). Some scientist estimate that life would appear on more than 10% of such Earth-like planets. If we use, again, a superconservative estimate, say 0.01%, instead of 10%, then the number of planets with living things is one trillion. One trillion planets with life! And remember, this is a super-super underestimated number. I hope this is more than enough to convince you that we are definitely not alone in this vast Universe.

Of course, this estimate does not mean there are one trillion different kinds of intelligent "aliens" with advanced technology. Most life on those planets should be very primitive form, like bacteria, or they may have become extinct already. It would be highly unlikelyh for us to have any communications with intelligent extraterrestrial organisms in any foreseeable future. In fact, the White House recently acknowledged these views in their official response to a petition (!/response/searching-et-no-evidence-yet).

But scientist are still hopeful for the possibiity of extraterrestrial life even within our solar system. Mars, our next door, is a promosing candidate as a life-habitable planet, and NASA just recently launched it new "Mars Science Laboratory" mission on November 26. The mission includes a new six-wheel rover called "Curiosity", which is supposed to arrive at Mars in August next year and then do various explorations and experiments to look for traces of life on the Red Planet. Other possible places for life within our solar system include Europa, one of Jupiter's moons that are believed to have a huge liquid water ocean under its ice crust, and Titan, the largest moon of Saturn that has a thick atmosphere and lakes and rivers made of liquid methane. The mechanisms of possible life in such extreme environments may be quite different from what we know about life on Earth. To prepare ourselves to be able to recognize unfamiliar living things when we encounter them on other planets or moons, some scientists are exploring various possibilties of life, or "life as it could be", to expand our scope and definitions of living things.”


Last Updated: 3/1/17