Does Intelligence Depend on Star Color?

Our civilization on Earth resides around a yellow dwarf star, but orange and red dwarf stars are much more numerous in the galaxy. Does this suggest that we should orient the search for extraterrestrial intelligence (SETI) toward these smaller and redder star systems? Or are certain star types better abodes for intelligent life than others?

Myself and Ravi Kopparapu address these questions in a chapter titled, “The Drake equation as a function of spectral type and time,” which appears in the book Habitability of the Universe Before Earth. Our approach invokes the Drake equation, which is a probabilistic device for estimating the number of communicative civilizations in the galaxy. The conventional terms of the Drake equation are the rate of star formation, the fraction of stars with planets, the number of habitable planets per system, the probability of life, the probability of intelligence, the probability of communicative technology, and the average lifetime of such civilizations. The product of all these terms yields the number of civilizations in the galaxy that SETI could conceivably discover.

Our book chapter analyzes calculations of the liquid water habitable zone for plants orbiting yellow, orange, and red dwarf stars. These habitable zone calculations provide constraint on the terms of the Drake equation. In particular we consider the dependence of each term of the Drake equation on both the stellar type (color) of the star, as well as the change in each parameter over time since the galaxy formed. We suggest that the habitability of red dwarf systems may peak in the far future, while the present time is optimal for habitability around yellow and orange dwarf stars.

Should We Transmit Into Space?

Deliberate and unintended radio transmissions from Earth propagate into space. Deliberate transmissions are intended as attempts to send messages to potential extraterrestrial watchers (known as “Messaging to Extraterrestrial Intelligence”, or METI). Unintended radio leakage includes television and radio broadcasts, cell phone networks, and high-power military and astronomical radars. This radiation gives evidence of our technological civilization to any extraterrestrial watchers.

Do radio transmissions pose a risk and should they continue? I recently published a paper with a team of BMSIS scientists in the journal Space Policy that examines the various benefits and harms that could arise as a result of human transmissions into space. In general, we think that the value of radio communication on Earth today is too large to justify ceasing all radio transmissions in order to reduce the risk of being found by a hypothetical harmful extraterrestrial civilization.

Most deliberate METI transmissions are detectable over much smaller volumes than the radio leakage. These transmissions are either short in duration or use a high bandwidth, in contrast to television carrier waves or high-power radars. These transmissions do not increase the probability of contact with extraterrestrial civilization. Such METI attempts are also valuable for education and public outreach efforts on Earth and for developing scientific groundwork for future METI projects. Given the modest costs associated with METI at low levels of detectability, we think that such projects should continue.

In contrast, high-power and persistent METI projects could have detectable volumes greater than the radio leakage, and would have a greater probability of being detected by any extraterrestrial watchers. The consequences of contact with extraterrestrials are highly uncertain, so we cannot say with confidence whether or not such attempts at METI should proceed. One additional benefit of transmitting messages into space at high power is that they serve a purpose analogous to digital time capsules, preserving the knowledge of human civilization, should our species become extinct. This may provide an additional justification for engaging in METI; however, the cost of maintaining such a long duration beacon must be weighed against the long-term benefits.

Existing governing structures or treaties are currently lacking for METI. Active engagement in long-term METI would benefit from international cooperation in order to accurately represent Earth and humanity and to better understand how to communicate effectively with an unknown observer.

Artifacts in the Solar System

One way that astronomers and astrobiologists search for life in the galaxy is observation of rocky planets orbiting other stars. Such planets may contain an atmosphere, liquid water, and other ingredients that are required for biological life on Earth. Once a number of these potentially inhabited planets have been identified, the next logical step in exploration is to send remote exploratory probes to make direct observations of these planets. Present-day study of other planetary systems is so far limited to remote observation with telescopes, but future plans for exploration include the design and deployment of small robotic exploratory spacecraft toward other star systems.

If intelligent, technological extraterrestrial life exists in the galaxy, then it is conceivable that such a civilization might embark on a similar exploration strategy. Extraterrestrial intelligent (ETI) civilizations may choose to pursue astronomy and search for planets orbiting other star systems and may also choose to follow-up on some of these targets by deploying their own remote exploratory spacecraft. If nearby ETI have observed the Solar System and decided to pursue further exploration, then evidence of ETI technology may be present in the form of such exploratory probes. We refer to this ETI technology as “non-terrestrial artifacts”, in part to distinguish these plausible exploratory spacecraft from the flying saucers of science fiction.

In a recent paper titled “On the likelihood of non-terrestrial artifacts in the Solar System”, published in the journal Acta Astronautica, myself and co-author Ravi Kopparapu discuss the likelihood that human exploration of the Solar System would have uncovered any non-terrestrial artifacts. Exploratory probes destined for another star system are likely to be relatively small (less than ten meters in diameter), so any non-terrestrial artifacts present in the Solar System have probably remained undetected. The surface and atmosphere of Earth are probably the most comprehensively searched volumes in the Solar System and can probably be considered absent of non-terrestrial artifacts. Likewise, the surface of the moon and portions of Mars have been searched at a sufficient resolution to have uncovered any non-terrestrial artifacts that could have been present. However, the deep oceans of Earth and the subsurface of the Moon are largely unexplored territory, while regions such as the asteroid belt, the Kuiper belt, and stable orbits around other Solar System planets could also contain non-terrestrial artifacts that have so far escaped human observation. Because of this plenitude of nearby unexplored territory, it would be premature to conclude that the Solar System is absent of non-terrestrial artifacts.

Although the chances of finding non-terrestrial artifacts might be low, the discovery of ETI technology, even if broken and non-functioning, would provide evidence that ETI exist elsewhere in the galaxy and have a profound impact on humankind. We do not argue that the search for non-terrestrial technology should be given priority over other astronomical missions; however, as human exploration into the Solar System continues, we may as well keep our eyes open for ETI technology, just in case.

Sending Messages Into Space

One possibility for communicating across the vast distances of space is the use of radio or other electromagnetic waves. Human civilization already posses the technology to broadcast and receive signals at many wavelengths. If other extraterrestrial civilizations exist in the galaxy, then it is possible that they could develop similar capabilities. Based on this premise, the search for extraterrestrial intelligence (SETI) has scanned the sky for over fifty years now to look for any such signals. Along similar lines, a handful of attempts at messaging to extraterrestrial intelligence (METI) have been undertaken in recent years, with hopes of being picked up by an extraterrestrial listener. The content of these messages has increased in complexity and content, though, which may produce cryptic messages that are disorganized or difficult to decipher.

In a recent paper published in the journal Space Policy, my co-authors Dimitra Atri and Julia DeMarines and I propose the development of a METI protocol in order to guide the construction and transmission of messages to extraterrestrials. A METI protocol would include technical considerations such as the method of signal encoding, message length, and transmission strategy. This protocol would also provide guidelines for the content of messages, which includes limits on culturally-dependent, anthropocentric, or sense-dependent information. This will help ensure that a message into space is more representative of Earth as a whole and may also increase the likelihood that the message is understood by potential listeners.

As a way of testing messages and promoting educational outreach, we will implement an interactive website in which users can attempt to submit or decrypt messages according to a METI protocol. This will allow messages to be tested across cultural borders, which arguably is a minimum requirement for a message that would be sent to unknown extraterrestrial listeners. Such an exchange will also help users of the website to gain insight into cultures other than their own by discovering success or failure at effectively communicating a message to unknown receivers on Earth.