ast 309L final test

What is so remarkable about the Trappist-1 planetary system?

It consists of seven Earth-size transiting planets in a compact orbital configuration

What is/are examples for possible biosignatures?

All of the above

Our best hope for determining that life on an extrasolar world lies i

Obtaining spectra that allow us to determine atmospheric composition

Why do most astronomers think that the search for biosignatures and not SETI will more likely lead to the first detection of probably extraterrestrial life?

Because even life on Earth was microbial for most of its time

The complete version of the Drake equation allows us to estimate the number of

intelligent civilizations capable of communicating in our galaxy now

Which of the following factors in the simplified Drake equation is best known?

Ne the number of habitable planets in our galaxy

Which of the following factors in the simplified Drake equation is least well known?

L fraction of civilization-bearing planets that have intelligent civilizations on them now

Suppose there are a million habitable planets in our galaxy, that one in ten are habitable, that one in a thousand planets with life had at some point had an intelligent civilization, and that one in a hundred civilizations that has ever existed is in existence now. Given this, how many civilizations are in existence now?

1

Given that life on Earth appears to have arisen quite easily and rapidly suggests that the factor fl, in the Drake equation could be close to

1

Which of the following cases in the Drake equation would be the minimum necessary condition for us to be the only intelligent civilization currently in our galaxy?

any one of the factors to be zero

The factor L, in the Drake equation depend

how long civilizations survive once they have arisen

The ratio of an animal's brain weight to body weight is thought to be roughly proportional to it

intelligence

In the Drake equation, a value of flife that is close to one and a value of fciv that is close to zero would imply

Primitive life is common but intelligent life is rare

Suppose that we discover life on Mars with an origin independent of life on Earth. Which term in the Drake equation would be most affected?

fl

The range of frequencies that are contained in a particular radio signal is referred to as its

bandwidth

Which of the following signals from an advanced civilization would be easiest to detect?

high intensity, narrow bandwidth

Approximately how soon might we expect an immediate response from a civilization that received the signal we sent to the globular cluster M13 (21,000 light-years away) in 1974

in 42,000 years

On the basis of the scientific method, which of the following would be considered conclusive evidence that we are being visited by aliens?

none of these would be conclusive

Scientists acknowledge that even in careful studies, they have been unable to explain about 10% of UFO sightings. Given that fact, why don't scientists generally accept that UFOs are alien spacecraft?

There are many possible explanations for the unexplained sightings besides alien spacecraft

Traveling at their current speeds, how long would it take the Pioneer and Voyager probes to travel the distance to the nearest stars?

at least 100,000 years

Which is the most common type of spacecraft propulsion used today?

chemical rockets

The Luyten 726-8 binary system is approximately 9 light-years away. If we send a spacecraft to visit this system traveling at 90% of the speed of light, how long will a one-way trip take as measured from the Earth

10 years

How is the "Fermi Paradox" normally stated?

Where is everybody?

Given how many times intelligent civilizations could have appeared over the universe's history, it is most likely that if other civilizations exist, they will

be much more advanced than us

Suppose that a civilization is never able to build spacecraft that can travel faster than a few percent of the speed of light. Could they still colonize the galaxy? Why or why not?

Yes, in principle, within as little as about ten million years

Try the following sample numbers in the Drake equation (Number of Civilizations = Ne x flife x fciv x L). Suppose there are 1000 habitable planets in our galaxy, that 1 in 10 habitable planets has life, that 1 in 4 planets with life has at some point had an intelligent civilization, and that 1 in 5 civilizations that have ever existed is in existence now. How many civilizations would exist at present?

5

Drake Equation Variable: N

The number of civilizations in our galaxy with which communication is possible

Drake Equation Variable: R*

The average rate of star formation per year in our galaxy (2017: 1.5-3/yr)

Drake Equation Variable: Fp

The fraction of those stars with planets (2017: approaching 1)

Drake Equation Variable: Ne

The average number of those planets that may develop an ecosystem (2017: about 0.2-0.4)

Drake Equation Variable: Fl

The fraction of those planets that succeed in developing life

Drake Equation Variable: Fi

The fraction of those planets with life that develop intelligent life

Drake Equation Variable: Fc

The fraction of those planets with intelligent life that develop interstellar communication

Drake Equation Variable: L

The average length of time such civilizations survive and continue to send communications (MOST IMPORTANT TERM)

What is the purpose and usefulness of the Drake equation?

The Drake equation is a probabilistic argument used to estimate the number of currently active, communicative, extraterrestrial civilizations in the Milky Way galaxy. It was first written down by Frank Drake in 1961 to stimulate discussion on SETI at the first meeting at the Green Bank radio telescope.

In order for our Drake equation “N”to come out reasonably large (say a million), the average lifetime L must be extremely large, something like

100,000 to a million years

Why is L the most important term in SETI?

Given the uncertainties in the other terms, L becomes the dominant term. N and thus the probability of success of SETI is controlled by L.

What terms of the Drake equation do we have data for?

R*, Fp, Ne

What terms of the Drake equation are still very uncertain?

Fl, Fi, Fc, L

What is SETI?

the attempt to measure N of the Drake Equation or at least find N>1

What is the main strategy of communication for SETI?

Signals from extraterrestrial civilizations (intentional or unintentional)

What is the only instance of SETI transmission?

Arecibo: the 1974 signal to globular cluster M13

How would an alien civilization try to communicate across many light years of space?

They will use photons

Why have most SETI searches concentrated on radio wavelengths instead of optical?

Interstellar dust, radio photons are cheaper, and noise

What was humanity's first attempt to detect interstellar radio transmissions?

Ozma

What did the attendees at the Green Bank meeting call themselves?

The Order of the Dolphin

What is the largest ever scientific research program aimed at finding evidence of civilizations beyond Earth?

The Breakthrough Listen Project

What is a technosignature?

Megastructures (Dyson spheres) and products of technological activity (e.g. gamma rays from their (hypothetical) fusion propulsion systems or CFC molecules from their air conditioners)

What is a Dyson sphere?

A spherical shell at same distance from star as the home planet. The intercepted energy is somehow channeled to planet. But shell is heated by the incident radiation.

Artifact SETI or Technosignatures

Energy-hungry civilizations might use a significant fraction of available starlight to power themselves. Energy is never “used up”, it is just converted to a lower temperature. If a civilization collects or generates energy, that energy must emerge at higher entropy (e.g. mid-infrared radiation). This approach is general: practically any energy use by a civilization should give a star (or galaxy) a IR excess.

Current status of technosignature projects

Tabby's Star: not a Dyson Swarm, definitely not aliens, no evidence of technology-related radio signals from the star system, transiting planet with large ring system, comet swarm, asymmetric dust ring, disintegrating planet/asteroid/comets, unusual magnetic activity

What are the differences between classical SETI and Artifact SETI?

Artifact SETI is hard-pressed to prove phenomena cannot be natural. Communication SETI must cast an impossibly wide net. They are complementary.

What are the major obstacles to interstellar travel?

Enormous distances to even the nearest stars

Interstellar probes and messages

Pioneer 10 and 11: Pioneer Plaque (hyperfine transition of neutral hydrogen, relative position of the Sun to the center of the Galaxy and 14 pulsars)

Voyager 1 and 2: Voyager's Gold Disk (images, sounds, speech, music)

Possible solutions to Fermi Paradox

They are here, we are aliens, Zoo Hypothesis, they can't communicate with us yet, they don't want to talk to us, aggressive civilizations are looking for signals so ET is keeping low, different mathematics, THE GREAT FILTER, we are the first, habitable planets are rare, life is rare, complex life is rare, intelligent life is rare, science & technology is not common for intelligent life

Interstellar visitors

Oumaumua, Comet @I/Borisov