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AstronomyAnswerBook: Life

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1. Life ... 1.1. Evolution ... 1.2. Primordial Soup ... 1.3. Star Dust ... 2. Life Elsewhere in the Universe ... 3. Life On An Earth That Is Ten Times As Large ... 4. How to Live On Another Planet

This page answers questions about life. The questions are:


1. Life

Fossils are remains of things (such as plants and animals) that lived long ago and that have turned to stone after the organism died. If you study these fossils, then you learn about the history of life on Earth. It turns out that the further back in time you go, the simpler the organisms get. If you go back a few thousand million years, then all organisms consisted of just a single cell, while the plants and animals of today are made up of millions or thousands of millions of cells.

A cell in a living organism on Earth is made up of very many different molecules, of which some are simple (such as water molecules) and some are very complicated (such as proteins and DNA which are made up of amino acids which themselves are complex molecules). Those complex molecules are made by living organisms themselves. Nowadays, we can make some of them in a laboratory or factory.

To understand the development of life on Earth, you must be able to answer the following questions:

  1. How did the plants and animals and people of today, with thousands of millions of cells, follow from the simple single-celled organisms of long ago?
  2. How did the complex molecules that make up the first living cells come together to form a cell?
  3. How did the complex molecules form out of the loose atoms and simple molecules that were available on the desolate Earth?
  4. Where did those atoms come from?

1.1. Evolution

Scientific investigation has shown that life on Earth develops through evolution. This works roughly as follows: A child has many characteristics. Some characteristics are easy to see (such as the color of hair, eyes, and skin, or the shape of the nose or fingers), and some are not (such as how easily the child gets ill, or how easily it gets lost, or how considerate it is). Some of those characteristics come from the parents (so children often resemble their parents), some are taught, and some are a surprise.

The adults that are best adjusted to their surroundings tend to have the most success in life (for example, they have plenty to eat, have good health, and so on), so they can usually raise the most children. Because children inherit some of the characteristics and circumstances of their parents, children of successful parents tend to have a better chance of success themselves. Adults that are badly adjusted to their surroundings (so they lack food, or are frequently ill, or die young) tend to raise few children or no children at all.

If the circumstances change, then different characteristics may bring success in life. For example, if the climate gets colder, then it becomes important that you can stand the cold, so then people who can stand the cold better will have more success and more children than people who cannot stand the cold. The children's generation will then have a greater proportion of cold-resistant people than the parents' generation had. In this way, each generation is on average slightly different than the previous one, if the environment changes. All of those small average changes from one generation to the next add up, so if you go back far enough in time, then the people of that time will look quite different from the people of today.

In this way, with small average changes from generation to generation, the people of today have evolved from apelike ancestors of millions of years ago, and those came, just like all other mammals, from reptile-like ancestors, and those from amphibian ancestors, and those from fish-like ancestors, and so on, until a very long time ago all living organisms contained just a single cell each.

1.2. Primordial Soup

We don't know a lot of details of how the first single-celled organisms formed. It is a hard topic to investigate, because the conditions on Earth are very different today from what they were at that distant time. For example, at that time there was no oxygen in the atmosphere, the land was totally barren, and the seas were probably not quite as salty as they are today. Because there was no oxygen in the atmosphere then, there was no ozone layer (ozone is a special form of oxygen) to keep out much of the harmful ultraviolet radiation from the Sun (that can cause sunburn on pale skin). That radiation reached the land and upper layer of the sea at full strength and could tear apart simple and complex molecules, which could then sometimes rearrange in novel ways to form new molecules.

Scientists have investigated which simple molecules and atoms might have been in the atmosphere and the sea then. When in 1953 Harold Miller put only such simple molecules (hydrogen, methane, ammonia, and water) together in a container and then sent an electric spark (like lightning) through it, then he found that complex molecules such as amino acids had formed. Apparently, it is not always difficult to form the kinds of complex molecules that living organisms are made up of. Other experiments have shown that forming something that looks like the membrane that is the surface of a cell is not uncommon in the right circumstances.

All in all, scientists have not (yet) been able to create a living cell from just simple building blocks and natural sources of energy (such as lightning), but it seems clear that life must have formed in a sort of "primordial soup" of simple molecules where unusual chemical reactions sometimes occurred as a result of lightning or ultraviolet radiation or volcanic heat, which formed complex molecules. The science that investigates the formation of life on Earth and on other planets is called exobiology. Search for that word (or for primordial soup) on the internet.

1.3. Star Dust

All molecules are made of atoms. There are about 90 kinds of atoms in nature, which correspond to the chemical elements such as hydrogen, oxygen, carbon, calcium, silicon, sodium, iron, and gold.

Models of the Big Bang show that during the formation of the Universe only the lightest elements hydrogen, helium, and lithium were made. All other elements were made later, inside of stars. Stars are made up of mostly hydrogen and helium, which were formed already during the Big Bang. All energy (light, heat) that a star emits is produced in nuclear reactions inside the star. In those nuclear reactions, light atoms are combined into heavier atoms, and this produces some energy. Most stars send part of their material into space at the end of their life, as a planetary nebula, nova, or supernova, and in that way the heavier atoms that the star has produced can mingle into clouds of gas from which new stars and planets are made.

In this way, all carbon, oxygen, nitrogen, calcium, iron, and all other elements except hydrogen from which all living things are made up were once formed inside stars. You're made partially of star dust!


2. Life Elsewhere in the Universe

In my opinion there can be life elsewhere in the Universe. I think that most astronomers share that opinion (but I don't know that for sure). The Earth is not so scientifically unusual that there can only be life here. It is difficult to come up with a good estimate of the chance that there is life elsewhere in the Universe. If the chance that an arbitrarily chosen planet has life is very small, then you must look at very many planets before you can expect to find one that has life. Are there enough planets in the Universe for that? See

So far, we haven't found convincing evidence for life elsewhere in the Universe, but it isn't easy to think of simple ways to detect signs of life that you can use from Earth and that don't also have a non-living explanation. There are so many stars that it takes a long time before you have checked all of them for such signs of life. See


3. Life On An Earth That Is Ten Times As Large

We do not know any earth-like planets with a diameter ten times as large as that of the Earth. The only planets we know of that are that large are jupiter-like planets, with very thick and dense atmospheres of thousands of kilometers thick. If such planets even have a rocky core that you could stand on, then the pressure and temperature at the surface of their rocky core are most likely far too high for life to exist.

For equal mass densities, the gravity at the surface of a planet is proportional to the diameter of the planet. This means that the gravity at the surface of an earth-like planet with ten times the Earth's diameter is about ten times greater than on Earth. Creatures would need to be very sturdy to withstand such gravity. I would expect them to have very thick legs and to be low (not tall).

Another result of the stronger gravity would be that the planet would be able to hold on to its original atmosphere better, similar to Jupiter, so such planets would probably have a very thick hydrogen atmosphere anyway, with high surface temperature and pressure.


4. How to Live On Another Planet

If you wish to live outside on another planet, such as Venus or Mars, then you have to make it suitable for humans first. Changing a planet so it becomes habitable by humans is called "terraforming". It takes a lot of time. Some planets cannot be terraformed, because they have an atmosphere that is far too thick (such as the Jovian planets), or because they are far too cold (such as any planet further from the Sun than Mars is), or because they get far too hot (such as Mercury), or because their gravity is too strong so we'd be crushed, or because their gravity is too weak so the atmosphere would leak into space too easily (such as the Moon), or because they do not have essential elements or minerals near their surface where we can mine them and use them to produce oxygen gas or water or any of the other materials that we need.

There is no need to worry about the effects of gravity on Venus or Mars: On both planets you'll weight somewhat less than you do on Earth. See question 82.


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Last updated: 2016−02−07