Julian Date: 2459052.16
2019-2020: CLXXXI
THE DAILY ASTRONOMER
Tuesday, July 21, 2020
Remote Planetarium 68: Exo-Planets IV - Finding Aliens
As recently as thirty years ago, the existence of planets outside our solar system still remained hypothetical. The issue neatly divided the astronomical community into the optimists who subscribed to Giordano Bruno's then heretical remark that an "infinite number of worlds"populate the Universe and the pessimists who suspected that planets were quite rare. According to the latter group, planetary formation resulted from infrequent close passages of stars. The mutual tidal forces drew out their gases which then coalesced to form planets. The former group believed that planet formation was a natural consequence of star formation. Planets arose from the residual gases, dust and other debris that enveloped around them. Now, in 2020, we can safely assume that the optimists were correct. Astronomers have discovered more than 4000 planets -4295 as of this date since the early 1990s. Based on these detections, astronomers estimate that the galaxy might contain more than one trillion planets, forty billion of which are potentially habitable. By "potentially habitable," they mean that the planets are "Earth-like" worlds within their parent stars' habitable zones. We should mention that the term "Earth-like" can be misleading. According to this definition, an Earth-like planet is small, rocky and dense. Each of our solar system's four inner planets -Mercury, Venus, Earth and Mars- could all be described as "Earth-like." Three of these planets -Venus, Earth and Mars- are located within the Sun's habitable zone. A 'habitable zone' is one in which conditions could allow water to remain a liquid on the planet's surface. This definition presupposes that water is as essential to other beings as it is to us humans. This assumption is one that we believe to be safe...until we find out it isn't.
The determination that planets abound in the galaxy and that many of them are potentially habitable bodes well for those Earthlings who hope the Milky Way teems with life. Again, however, we find yet another divide separating the optimists and pessimists. The optimists assert that the Universe is so prodigiously creative that life must proliferate through the cosmos wherever conditions are conducive to its development. The pessimists argue that perils to life are as abundant as planets. After all, examine our own neighborhood: asteroids have pummelled our planet through the aeons; harsh cosmic radiation constantly washes over Earth; and the geological record contains numerous instances of mass extinctions. Life might begin in many places, but the probability of life progressing through the evolutionary process to an advanced state is highly improbable. "We've been lucky," the pessimist would insist.
Well, as was true with the planet issue, the matter of other life won't be resolved until we detect evidence of life. We should point out that we will NEVER be able to prove that alien life doesn't exist. Even if thousands of years pass without a single detection, alien races could still exist and in abundance. They just might be undetectable or too far away for us to ever find them. (How could we possibly find a fern covered planet populated by spores and lemurs tucked away in the Andromeda Galaxy?)
How can astronomers try to detect life on other planets since even the closest exo-planet, Proxima Centauri b, is 4.2 light years away? We know technological limitations currently preclude interstellar travel. Our most distant robotic probe, Voyager 1, is only 20.6 light hours from the Sun and it was launched 43 years ago! The farthest any human has ever gone from Earth is 248,655 miles, or 1.33 light seconds.* Our race hasn't exactly reached the Star Trek phase of our technological development yet. For this reason astronomers must do what astronomers have always done: make their discoveries through the analysis of radiation.
RADIO SIGNALS?
Searching for radio signals has been the aim of the SETI (Search for Extra Terrestrial Intelligence) since its founding in 1984. The theory is that an alien race could, like humans, have developed radio communication technology. Many of our radio signals propagate through outer space at light speed and so could be detectable by beings on worlds within the signals "sphere of propagation." For instance, radio waves from a 1970 broadcast are still moving away from Earth along a sphere extending fifty light years from Earth. One hundred and thirty-three stars exist within this fifty-light year radius. It is conceivable that extraterrestrial astronomers might be able to detect these waves. Due to the high attenuation, they couldn't actually listen to the program. Instead, they could discern "strange" waves exhibiting signs of manipulation, unless the indistinct radio energy that pervades the cosmos.
SETI's aim is to detect such radio signals from other parts of the galaxy, evidence of the existence of an alien race. The limitation of this method is that it requires those races to have advanced sufficiently so as to have been able to develop radio technology. Life first appeared on Earth about 3.7 billion years ago. Different forms of radio technology first developed in the late 19th century. The span of time between life's inception and the period when we were first able to "announce" ourselves to the Universe well exceeds three and a half billion years! If aliens were trying to detect signals from our solar system a couple of thousand years ago when Rome was at its apex, they wouldn't have found anything at all.
CHEMICAL TRACES?
Another possible method of extraterrestrial life detection involves detecting chemical "life signatures" from the atmospheres of Earth-like planets within their parent stars' habitable zones. Star light emitted by the parent star passes through the exo-planet atmosphere. The gases within the atmosphere absorb some of that light before it continues through outer space. Gases absorb light at various frequencies. Each type of molecule absorbs light at a specific number of wavelengths. By analyzing the spectrum of this light, astronomers can identify the chemicals present within the atmosphere.
The graphic above shows simulated chemical signatures of various types of exo-planets. [Note: Archean Earth refers to Earth during the "Archean Eon" between 2.5 - 4 billion years ago.] Astronomers seeking evidence of life on other worlds are looking for certain chemicals that might be indicative of metabolic activity on the planet's surface. These include:
- water vapor
- methane
- carbon dioxide
- molecular oxygen
We know that such gases are necessary for life to persist on Earth. We assume -perhaps incorrectly- that life on other worlds would leave such traces within the atmospheres of their home planets.
The search for extraterrestrial life continues. That astronomers haven't found anything yet shouldn't concern alien lovers. The number of star systems to be searched is vast. However, by knowing which star systems harbor Earth-like worlds within the habitable zones at least narrows the search slightly. We do know that the age-old question "Are we alone?" could one day be answered in the negative. By the nature of science, it will never be answered in the positive.
* Jim Lovell, Fred Haise and John Swigert attained this distance when their ill-fated Apollo 13 mission brought them all to the far side of the moon.
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SKYWATCHING TIP FOR THE DAY: Don't forget to look for Saturn!
Saturn reached opposition yesterday. Tonight, Saturn rises at 8:07 p.m. and will remain visible for the rest of the night. Find it in the eastern evening and western morning sky. At magnitude 0.1, Saturn is slightly brighter than Vega. Unfortunately, its magnificent ring system is not resolvable with the unaided eye. All the same, Saturn is well worth a look this week!
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