We know of only one place in the Milky Way where life exists. However, with hundreds of billions of stars, one would have to wonder if life exists elsewhere.
How does one start, considering we have not one confirmed "Hello" for a celestial neighbor? The answer is to look for conditions where the possibility of life (as we know it) can exist. Life works by chemistry ... organic chemistry ... which is really the study of 6 elements - Hydrogen, Carbon, Oxygen, Nitrogen, Phosphorus, and Sulfur. Of these elements, carbon is the key because it is the "glue" that allows for the complex bonds that make up proteins and amino acids. In addition, water is another key to life. It is water that promotes and facilitates most of the chemistry that keeps everything alive. Could life work without these keys? Certainly but why reinvent the wheel? We know that life works with this formula so why not start there?
Another thing we need is time (to make interesting life). If we use the earth as a model, we know that very simple life got started very early in our history. Soon after the earth formed and started to cool, water was present and simple one celled life formed (no one knows how that happened). For most of earth's history (over 3 billion years), life was extremely simple - one celled organisms. Things got interesting about 700 million years ago when multi-cellular life first appeared. From there, sexual reproduction was invented and things really took off.
Now lets assume that we need these key ingredients on a planet ... around some other star system. Where do we look? Below are some questions that might help you piece things together. Your job is to answer one of these questions and post the reply to this message. If someone has already answered the question, it is off the list and you have to choose another. So it is very important that you make it clear which question you are answering. You can do that by renaming the title window to the question letter ... this way all your fellow students will know exactly which questions have already been answered. Also, make sure you are replying to this message and not someone else's post.
Here are short answers for brevity. I would expect a bit more in student posts.
Question A - Life on earth can exist in many extreme environments. They are often called Extremophiles. It makes us aware that life elsewhere in the universe may also exist under conditions we consider inhospitable. Give 3 examples where life is found on earth in extreme conditions.
Biologists have found bacteria thriving in ice, deep underground, in boiling water, in acid, and even nuclear reators.
Question B - When discussion this topic, astronomers usually refer to a region around a star as the "habitable zone" or "zone of life". Exactly what are they referring to and does this "zone" ever change with time? If so, please explain how the "zone" around our sun might change as the solar system ages.
The habitable zone around a star is a region where life can exit. Traditionally this corresponds with the region where liquid water is stable. All stars have such a zone but it depends on several variables. The most important is the luminosity of the star. The habitable zone around dim stars is closer to the star and more luminous stars have habitable zones further out. As a star ages, its luminosity will increase. As a result, so does the habitable zone. Astronomers now understand that the habitable zone can exit in some unusual places - under the ice of Jupiter's moon, Europa (for example).
Question C - Most stars are part of a binary or multiple star system. Is it possible for habitable planets to exist if the system contains 2 (or more) stars? If so, what are those conditions (please list your sources)?
Planets in a binary star system are called circumbinary planets and several have been discovered. Can such a planet support life? The short answer is YES. There are two types of circumbinary planets: S-type and P-type and without going into too much here, just look here for a short overview.
Short answer: If two stars are very, very far apart, a planet can orbit just one of those stars and support life (S-type). If two star are very, very close together, a planet can orbit both stars and still support life (P-type). This was depicted in Star Wars on Luke's home planet Tatooine.
Check this out.
Question D - Why is it unlikely that we would find life around type "O" or "B" stars ... and even more unlikely we would find intelligent life there.
These stars emit most of their radiation in the UV which can destroy organic molecules. Life may be shielded from this danger if it existed deep under water. As far as intelligent life goes - forget about it. Assuming it takes some amount of time to progress to an intelligent stage, these stars do not qualify since they only exist on the order of million (not billions) of years.
Question E - There are a lot of low mass stars (as you learn in this unit). Give at least one reason why you might have problems finding conditions necessary for life around low mass stars (lower than the sun). One reason can be found in my web notes but there are others. This question may be answered by more than one student if you can come up with a different reason.
The good news is that low mass stars exist for a very long time. The bad news is the habitable zone would tend to be very narrow and close to the star. You would need a planet with almost no eccentricity (a perfect circle) to remain in the habitable zone. Another issue becomes tidal forces. Since the planet is forced to orbit so close to the star, it is likely to become tidally locked so only one side permanently faces the star and the other side is in perpetual night. Also, orbiting too close to the parent star makes the planet susceptible to damaging solar flares which can destabilize the planet's atmosphere and most low mass stars experience a lot of solar flares. But there may be hope. Read about Ross 128 b here.
Question F - Why is it unlikely life will exist near the center of the Milky Way? This question may be answered by more than one student if you can come up with a different reason.
Stars found in the center of the Milky Way are in a very hostile environment (at least to us). The central region is bathed in high concentrations of UV radiation (from young hot stars). Because stars are so physically close, stellar perturbations will significantly disrupt any star's Oort cloud .... causing excessive comet impacts on any planets around these stars. Nearby supernova explosions would likely disrupt any habitable planet. Also, the center of the Milky Way is the home of a supermassive black hole. If that isn't bad enough, astronomers believe there may be thousands of stellar mass black holes lurking in the center of our galaxy. All together, not a good place to live. It is also interesting to note that the abundance of "metals" (a term used by astronomers to indicate any elements above hydrogen and helium) is limited to the galactic disk. That is to say, as you move away from the galactic disk, the elements necessary for life drops dramatically. So don't look for ET in the galactic halo either.
Question G - The Drake Equation is often used when discussion this topic. Write out this equation and define all the terms in it. What are some estimates (by leading scientists) for the solution to this equation? Please list your sources.
This equation estimates the number of planets in our Milky Way with intelligent life forms capable of interstellar communications. It is quite long so I'll just direct you here. The discovery of habitable exoplanets have been a game changer from past estimates.
Question H - Why is it likely that we will make first contact with intelligent life via radio wave communication? This question may be answered by more than one student if you can come up with a different reason.
We have been inadvertently sending out radio "noise" since the invention of radio and television. This portion of the electromagnetic spectrum is easy and cheap to transmit. It can get through most of the gas and dust found in interstellar space. It would be nearly impossible for us to purposely shield our broadcasting in this region of the spectrum. It stands to reason alien civilizations would have the same problem. We are likely to hear their random chatter instead of a deliberate message directed toward us. Interestingly, some have pointed out that the advent of fiber optics cables, we are actually reducing our radio broadcasts in space. Maybe in 100 years we, too, will be nearly radio silent ????
Question I - Explain why a two way communication (via radio communication) between earth and intelligent extraterrestrials is not likely.
One word - distance. The round trip time to say hello and then get a reply is years (at minimum) and likely lifetimes to go from "how you doing?" to "ok, I guess".
Question J - What efforts are we currently taking to look for extraterrestrials? This question may be answered by more than one student if you can come up with a different reason.
We covered SETI in the last unit. Look at that answer.
Question K - Why is it unlikely we have been visited by extraterrestrials (from a physics point of view). This question may be answered by more than one student if you can come up with a different reason.
One word - distance. Radio waves travel at 186,000 miles/sec and we haven't heard a thing. Space ships travel much, much slower (until be invent the warp drive promised in Star Trek). We will hear the aliens well before we get visits by them. Now let's turn the table and say that microbes actually could be transported from one planetary system to another via comets or meteorites. It would require lots of time and dormant bacteria ... check out an idea known as Panspermia.
Another very acceptable answer is time. It takes several generations of stars to generate the heavier elements on the periodic table (via supernovae). To make another intelligent civilization, you need to first have a universe with the right building blocks to make their home world and that takes a lot of time. It then takes more time to make simple life into complex life. Stated another way, the universe may not be old enough to make a civilization capable of interstellar travel. We are very far from that step ourselves. However, I believe the way to go is to send probes (like how we explore our solar system). Robots have all the time in the world to make those kinds of trips. Using that same logic, look for a smart probe well before we get a visit from ET.
Question L - Is there any hope of finding life at Venus? Why (and where) or why not?
The surface of Venus is far too hot (at 860 degree Fahrenheit) to support any form of life . However, Venus has mysterious dark streaks in its clouds that may be microbial life suspended in the atmosphere, according to Sanjay Limaye of the University of Wisconsin. The conditions about 25 miles above the surface of Venus may be just perfect for microbes to exist. Read more here. OK, things really got interesting in September 2020 when a molecule called phosphine was discovered in the atmosphere of Venus. What is phosphine (PH3)? It is a molecule that is produced by bacterial life in an oxygen free environment. There are inorganic ways to produce phosphine but none of those conditions appear to be present on Venus. Read more here.
Question M - Are there any plans to look for life at any of the potential "subsurface ocean" moons around the Jovian planets? If so, what are they and how will they work?
Look for the Europa Clipper set for launch in the 2020's. The radiation from Jupiter is too harsh for a surface landing probe so this device will orbit Europa and do a detailed study to learn more about the water below the ice. In addition, it will sample material ejected from Europa (either by impacts or geysers) and determine its chemical make-up.