An artists' depiction of the member planets of the newly discovered Trappist-1 System. No one has actually observed these worlds.
In one media spot or article after another one beholds elaborate images showing the 7 planets discovered in the Trappist -1 system. This is an exoplanet system in the constellation Aquarius, about 39 light years away from Earth. Of course with all such finds, and especially this one, the media hype immediately goes to whether any of the worlds can support life. And so we saw and heard much breathless speculation in the past few days of life maybe emerging on one or all of the three planets, designated: e, f and g.
Let's first clear the air that despite the planetary images no one has actually observed these worlds. They were detected by ESO investigator Michaël Gillon, and his team of exoplanet researchers at the University of Liège in Belgium using the standard "eclipse" techniques. The basic principle at work is elementary to grasp and illustrated by the graphic shown below:
In this method, the exoplanet passes in front of its parent star producing a dip in the light curve over a defined interval t2 - t1(forming the dip). The length of the "dip" enables the investigator to deduce the presence of an orbiting planet about the parent star. To fix ideas, Gillon and his ESO team have been interested in TRAPPIST-1 since late 2015. Using the European Southern Observatory’s Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, they detected small dips in the star’s brightness at regular intervals. As I noted, such dips are incepted when putative planet crosses between the star and Earth in our line of sight.
Last May, the ESO scientists published their discovery in Nature: citing three rocky bodies, dubbed TRAPPIST-1b, -1c and -1d. Not long after the study was published, Gillon noticed that TRAPPIST-1d was behaving oddly. On taking a closer look with the Very Large Telescope, he realized that the dip in brightness he thought originated from 1d alone was actually caused by three planets, all transiting at the same time. In other words, three "sub-dips" in the single light curve of 1d gave rise to 3 planet indirect detections.
According to one co-author or the study, Julien de Wit, a planetary scientist at MIT:
"This happens only once every three years. The chance of catching it is less than one in a thousand. It’s funny because it’s such a huge paper with amazing results, and we got it from sheer luck.”
Luck indeed! And having the instruments and insights to make these detections that have given rise to our acceptance of this compact solar system.
Let's examine further extracted data from the light curves analysis.
The periods of the three habitable zone planets are: 6.1 days for e, 9.2 days for f and 12.3 days for g. These compare to 365 1/4 days for Earth. Why are the periods of revolution so very brief? Well, because all seven planets actually orbit their within a very confined region. If Trappist -1 was at the Sun's position, all seven worlds would be within the orbit of Mercury - the nearest planet of our system to the Sun. The reason the Trappist planets aren't fried is that their star is much cooler. Specifically, it's classified as an ultraviolet dwarf star - less than a tenth the size of the sun and about a quarter as warm.
So even granted that the Trappist planets are relatively close to this UV dwarf, we expect the conditions on these planets wouldn't exactly be called "balmy". In fact, I can't imagine any sober human who'd actually want to travel to any of them and look around, even the ones in the so-called "habitable zone" Why?
The Gillon team that conducted the investigations determined that the six inner planets are locked in an orbital resonance, meaning that lengths of their orbits are related by a ratio of whole numbers. Because of this, the bodies are likely “tidally locked.” In other words, the amount of time it takes a body to orbit matches the length of one rotation on its axis. This results in the same side of the planet always facing the object about which it orbits. For example, the moon is tidally locked with Earth, which is why we always observe the same face (for the most part, we actually see a bit more due to nutation), when we look up at night.
As I noted in previous posts about similar exoplanets, this means that one side of each body is constantly exposed to its sun's heat, while the other side is perpetually in darkness. This means that half of each planet freezes while the other half burns. The only region where one would find tolerable temperatures would than be at the terminator or the "line" separating day and night sides.
If one is going to do a "life search" on these Trappist worlds, I think they will be disappointed. As one recent article put it, "even if they turn out to be warm and wet, these worlds might not be great places to live". You think? Constant darkness and frigid cold vs. excess heat and constant light. Boil vs. freeze? Of course, there might be some form of primitive micro-organisms assuming there is water on any of the 3 "habitable" worlds. (Remember again when we use the term "habitable" we mean only in a potential sense, not that they actually are.)
As Elisabeth Adams, an exoplanet researcher at the Planetary Science Institute put it:
"The very idea of a “habitable world” is purely theoretical. Scientists have only one source of data on habitable planets, and that’s Earth. We don’t actually know the parameters that are needed for life on another world."
Indeed. So it is best not to jump too far ahead of what we actually know. That knowing must await (at least) further detailed analysis of the atmospheres of the e, f and g worlds of the Trappist -1 system.
Rather than hyping life on these worlds, a better take might well be acknowledging that Trappist -1's system presents an unprecedented window on how solar systems work. Thus, beyond the current data the each planet is more or less "Earth-sized", their varying densities and distances allow for detailed comparisons of the worlds.
This is a useful template that will be of inestimable benefit in further exoplanet system detections.