ASTRO TEXTBOOK NOTES
1.1 The possibility of Life Beyond Earth
Scientists are interested in aliens too, although most scientists remain deeply skeptical about reports of aliens on Earth.
Scientists are therefore searching for life elsewhere, looking for evidence of life on other worlds in our solar system, trying to learn whether we should
expect to find life on planets orbiting other stars, and searching for signals broadcast by other civilizations.
The discovery of life of any kind beyond Earth would forever change our perspective on how we fit into the universe as a whole.
When we search for extraterrestrial life, or life beyond Earth we are looking for any sign of life, be it simple, complex, or intelligent.
First we need to define what life is, before we begin to look for them.
Because of this definitional difficulty, the scientific search for extra terrestrial life in the universe generally presumes a search for life that is at least
somewhat Earthlike and that we could therefore recognize based on what we know from studying life on Earth
Until quite recently, however, all these ideas remained purely speculative, because there was no way to study the question of extraterrestrial life
scientifically. It was always possible to imagine extraterrestrial life, but there was no scientific reason to think that it could really exist
Prior to the twentieth century, for example, some scientists guessed that Venus might harbor a tropical paradise—a guess that was based on little
more than the fact that Venus is covered by clouds and closer than Earth to the Sun.
Mars was the subject of even more intense debate, largely because a handful of scientists thought they saw long, straight canals on the surface.
Today, we have enough telescopic and spacecraft photos of the planets and large moons in our solar system to be quite confident that no civilization
has ever existed on any of them.
But although large, multi cellular life in our solar system seems unlikely anywhere but on Earth, new discoveries in both planetary science and biology
have given us some reason to think that simpler life—perhaps tiny microbes—might yet exist on other planets or moons that orbit our Sun.
Also, while we’ve long known that the universe is full of stars, we’ve only recently gained con crete evidence telling us that it is also full of planets,
which means there are far more places where we could potentially search for life. Also, advances both in scientific understanding and in technology now make it possible to study the question of life in the universe through established
techniques of science, something that was not possible just a few decades ago.
While it remains possible that life exists only on Earth, we now have plenty of scientific reasons to think that life might be widespread and that we
could detect it if it is.
1.2 The Scientific Context of the Search
Even seemingly unrelated fields such as mathematics and computer science play important roles on the search for life in the universe.
Three particular disciplines play an especially important role in framing the context of the scientific search for life: astronomy; planetary science,
which includes geology and atmospheric science; and biology.
When you consider the dominance that this Earthcentered, or geocentric, view of the universe held for thousands of years, it becomes obvious
that astronomy plays a key role in framing the context of the modern search for life.
We are not the center of the universe in location, and we have no reason to think we are “central” in any other way, either.
By studying distant objects, we have learned that the physical laws that op erate in the rest of the universe are the same as those that operate right
here on Earth. This tells us that if something happened here, it is possible that the same thing could have happened somewhere else, at least in
Planetary science is the name we give to the study of almost everything having to do with planets.
By learning how planets form, we develop an understanding of how common we might expect planets to be.
But during the latter half of the twentieth century, a growing understanding of the processes by which our own solar system formed, gradually made it
seem more likely that other stars might similarly be born with planetary systems.
Much of this understanding is based on evidence obtained through human visits to the Moon and spacecraft visits to other planets
1995 was the year in which the first strong evidence was obtained for the existence of extrasolar planets, or planets orbiting stars other than our
Sun. Based on the statistics of these discoveries, it now seems likely that many or most stars have planets and, as we’ll discuss in Chapter 11, it seems
reasonable to imagine that life—and possibly even civilizations—could exist on at least some of these planets or their moons.
Graph shows the data led to OGLE2005BLG390Lb's discovery.
OGLE2005BLG390Lb is the name of a planet discovered in
Planetary science shapes the context for the search for life is by helping us understand how planets work.
For example, we now know why Jupiter is a gas giant and why Venus is so hot.
Planetary science also helps us understand what to look for as we search for habitable worlds—worlds that contain the basic necessities for life
We are asking whether it offers environmental conditions under which life could arise or survive, not whether it actually harbors life. (Microscopic life)
It would make no sense to search for life if we didn’t know something about how life functions.
The key question about the biological context of the search revolves around whether we should expect biology to be rare or common in the universe.
Observations make us confident that the basic laws of physics that we’ve discovered here on Earth also hold throughout the universe. We can be
similarly confident that the laws of chemistry are universal.
Could biology also be universal? That is, could the biological processes we find on Earth be common throughout the cosmos? If the answer is yes, then the search for life elsewhere should be exciting and fruitful. If the answer is no, then life may be a rarity.
Because we haven’t yet observed biology anywhere beyond Earth, we can’t yet know whether biology is universal. However, evidence from our own
planet gives us reason to think that it might be.
The mere presence of organic molecules does not necessarily mean that life will arise, but the history of life on Earth gives us some reason to think
that the step from chemistry to biology is not especially difficult.
The early origin of life on Earth makes it seem reasonable to think that life would emerge just as quickly on other worlds with similar conditions.
Biologists have found that microscopic life can survive and prosper under a much wider range of conditions than was believed only a few decades
The range of “right” conditions for life may be quite broad, so it might be possible to find life even on planets that are significantly different in character
Evidence that organic molecules