Ask Ethan: Was life on Earth possible anywhere else in our Galaxy?

Planet candidate for the emergence of life no doubt experiencing catastrophic events and periodic extinction. If life is destined to survive and develop, the planet must have certain conditions for its existence.

In our Galaxy there are hundreds of billions of stars, many of which have planets the size of the Earth, located at the right distance from their star so that they can have on their surface liquid water, so life has a chance to appear throughout the Milky Way. At least we think so. But can it not be so that our conditions on our planet somehow distinguish us and the life that appeared and developed on Earth? This is what the reader is asking us:

What would happen if our Solar System formed a little further along the sleeve of the Galaxy? If we were at the very end? And what if, instead of a huge black hole in the center, our solar system would be there? Would this change our climate? Could we survive?

Let's see what would change in such a case.


An illustration of a protoplanetary disk in which the planets and planetesimals create "grooves" in the process of formation. The outer disk provides material that eventually ends up in the mantles, crust, atmospheres and oceans of such planets as ours.

We are quite well aware of how and what has developed in our solar system over the past 4.5 billion years. A molecular cloud of gas with certain substances that enriched it — about 2% by weight of heavy elements, 28% helium, and 70% hydrogen — shrank and gave rise to new stars. One of them was destined to become our Sun, formed together with the surrounding protoplanetary disk, as happens in almost all stars.

For tens of millions of years, the hot Sun has evaporated the material of the inner part of the disk, and the outer, colder part fell inside and gathered around the people’s nuclei. The most massive giant worlds retained a large number of the lightest elements (hydrogen and helium), but the small rocky worlds did not succeed. All the rest was done by gravitational interactions, which determined today's image of the Solar System.



Many of the properties of the Earth and the Solar System seem special, but may not be necessary for the emergence of life. Unlike other rocky planets in the solar system, only the Earth has a Moon causing tides with ebbs and maintaining a steady inclination of the axis. Unlike many other star systems, there is a giant in ours - Jupiter - located just a little bit further from the asteroid belt. And, unlike most stars in the galaxy, we are located on the edge of the spiral arm spiral, 25,000 light-years from the galactic center.


The structure of our Milky Way is quite well-marked, including the position of our Sun. Today it is not known which stars and regions of the Galaxy are capable of supporting life.

For 4.5 billion years, life on Earth continued to survive and evolve, producing more and more complexity, diversity, and coding more and more information in DNA. We have experienced many mass extinctions, the causes of most of which are unknown to us. Although from 30% to 70% of all species of living creatures disappeared at one time or another, during the last period, since the fall of a giant asteroid 65 million years ago, life on Earth has not been interrupted. Time passed and biological activity continued.


The percentage of extinct species in different time intervals. The largest known one is the massive Permian extinction that occurred 250 million years ago, the reasons for which are still unknown.

But of all the properties of the Earth, which are absolutely necessary for life? What could lead to the emergence of the planet, the history of life on which will be different from ours, but life on it will be possible?

Until we find life outside the Earth, on planets outside the solar system, the answers to such questions will inevitably be purely speculative. But these are not empty guesses; these are theoretical statements issued with the help of the best science achievements to date. On the basis of all that we know, we believe that the conditions that allow life to exist are much more diverse and more flexible than most people would have imagined.


When the north pole of the Earth is maximally deflected from the Sun, it is maximally inclined towards the full Moon located on the other side of the Earth. The moon stabilizes our orbit, and slows down our rotation. It is not known whether the presence of such a moon is imperative for life to develop and maintain on the planet.

Take, for example, a large Earth satellite. Gravitational interaction with it maintains the stability of the axis of rotation of our planet. The current axis tilt is 23.5 °, but at very long intervals it varies from 22.1 ° to 24.5 °. In a world like Mars, the tilt of the axis is now almost the same as that of the Earth: about 25 °. But at intervals of tens of millions of years it changes ten times stronger than ours - from 13 ° to 40 °.

This leads to significant climate changes at different latitudes of Mars, surpassing any glacial periods on Earth. But as long as life can survive long-term temperature changes or migrate to more acceptable temperature zones, this should not be such a critical factor. Interestingly, the tidal forces of the moon also increase the duration of our day - from ≈ 8 hours to 24 hours in the last four billion years. And this, apparently, had no effect on life.


The motion of the asteroids of the main belt and the Trojan asteroids around Jupiter may be subject to the influence of a giant planet, but it is still unknown whether the number of asteroids crossing the Earth’s trajectory relative to the same stellar system increases or decreases with Jupiter, but without a similar gas giant.

This is very similar to the question of the presence of Jupiter in our Solar System. Yes, the conventional wisdom is that Jupiter “cleans” the asteroid belt and reduces the likelihood of an asteroid colliding with the Earth. But in fact, there are a lot of controversies on this issue. For example, think about this question: Does having Jupiter increase or decrease the likelihood that an asteroid will fly in our direction? Jupiter acts as a disturbing force, randomly giving additional speed to everything that passes by it. Many asteroids will be thrown out, but many stable ones can become potentially dangerous. We are still not fully sure whether this makes a positive or negative contribution to the cosmic equation of life.


The density map of stars in the Milky Way and the surrounding sky, where the Milky Way is clearly visible, large and small Magellanic clouds, and if you look closely, then NGC 104 to the left of the Small Magellanic Cloud, NGC 6205 is slightly higher and to the left of the galactic center, and NGC 7078 is a bit below. In general, the Milky Way contains from 200 to 400 billion stars, and the Sun is located about 25,000 light years from its center.

In addition, there are disputes about what stars are capable of supporting life. They do not just need to be not too massive and short-lived, but also, perhaps, they need to have a sufficiently large mass that exceeds a certain threshold. Most of the stars - about 80% - are red dwarfs. They are dull, quickly implementtidal capture of planets, and often emit large flashes. Is life next to them possible, or is a more massive, sun-like star necessary?

What about our location in the galaxy? About some things we can reason quite reasonably, for example, the presence of a sufficient number of heavy elements. To get rocky planets with ingredients that contribute to life, in our opinion, you need a sufficient number of heavy elements. Without them, only gas giants can appear, on which a variety of carbon compounds is impossible, without which life cannot be created.


A photograph of the galactic center at several wavelengths shows stars, gas, radiation, and black holes, as well as other sources. They collected a huge amount of material, including heavy elements and organic compounds, the necessary precursors of life. But there must be enough of them, or life will be impossible.

But what is this threshold? Is it necessary to have a very large number of heavy elements for everything to work out? And if they will be two times less than ours? And 10 times? And a hundred? We can mark out the number of heavy elements - what astronomers call metallicity- relative to the location of the star in the galaxy. And we may discover, unexpectedly, that if the stars are close to the plane of the Milky Way disk, and not too close, and not too far from its center, they will be more or less similar to ours. The necessary balance of heavy elements, if we assume that it is necessary to overcome a certain threshold for life, actually exists on most of the stars of the Milky Way that appear today.


The relationship between the location of stars in the Milky Way and their metallicity, that is, the presence of heavy elements. Stars from 3,000 light-years from the center of the Milky Way to several tens of light-years show an abundance of heavy elements, very similar to what is in the solar system.

Of course, there must be places where conditions are too harsh for life. A too massive star, 50% larger than the Sun, will not live long enough for life to achieve such complexity as on Earth. An inhabited planet too closely caught up in a cruel cataclysm — for example, from a supernova or a gamma-ray flash — can lose its life, although there is controversy over this, as life can survive this phenomenon. Where the density of stars is too high, the planet can simply be thrown out of its home star system or else it is critical to break its orbit. Where we are, the chances of such an event are very small, but closer to the center of the galaxy, they would have increased incredibly.


In the centers of galaxies there are stars, gas, dust, and black holes - and all this revolves around the central supermassive black hole. These masses not only react to the curved space, but also themselves distort space, as a result of which mutual gravitational interactions, resulting in descents from the orbits of stars and planets, turn out to be very frequent.

In order for life to develop successfully for billions of years, in our opinion, three main ingredients are needed: life must originate, conditions on the planet must be stable enough for life to continue, and the planet must avoid events that can destroy life for 100%. It is very easy to imagine such a planet as Mars, on which life began. But if the conditions of the planet change and turn into unacceptable for life, or if a catastrophe happens, due to which all living creatures die, then the world of earth type will not work.


Cataclysm events occur throughout the galaxy and throughout the universe, from supernovae and active black holes to the merging of neutron stars and others. Because of this, parts of space with a dense arrangement of stars may be too hard for life to appear, but in order to completely eliminate this possibility, we will need more evidence than we have now.

And yet there is little chance of the emergence of life only in the densest and most rarefied parts of the galaxy. The galactic center is inhabited by young, massive stars, next to which life is in the greatest danger; life on the most rarefied outskirts of the Galaxy would simply not have arisen. As far as we know, as soon as life appears on the planet and begins to inhabit it, it is very difficult to eradicate it.

We are quite confident that the conditions that existed on Earth after its appearance led to the emergence of a thriving biosphere, but it seems to us that completely different conditions could also lead to a similar result. In the great cosmic equation you should not discard the chances that life will survive and thrive in a huge variety of conditions. After all, the jungle, and hydrothermal springs, and the snow of the Antarctic - they are all filled with life. Another planet may not be suitable for humans, but it may just be suitable for aliens who grew up on it.

Ethan Siegel - astrophysicist, popularizer of science, blog Starts With A Bang! Wrote the books Beyond The Galaxy , and Treknologiya: Star Trek Science [ Treknology].