The TRAPPIST-1 System
(Originally posted February 23, 2017 on Blogger)
This blog will start out with a brief discussion of the TRAPPIST-1 system, and the discovery of five additional planets within it. However, my main goal with this blog is to share how exoplanets are found, as well as how their characteristics are learned.
Then I'll attempt to clarify what "habitable zones" are, and more importantly, what they aren't. I'll wrap up with a caveat to artists' renditions of exoplanets. Don't worry my friends, this is not a skeptic's blog! In fact, the TRAPPIST-1 system discoveries are absolutely incredible, and very exciting! It's only fair we are able to see this incredibly-exciting discovery without having to see it through the artificial lenses of rose-colored glasses.
TRAPPIST-1 SYSTEM INTRO
From 1997-2001, an all-sky astronomical survey was conducted by scientists from the Jet Propulsion Lab (JPL) in collaboration from scientists from the University of Massachusetts. Two 1.3-meter telescopes were used: one in Arizona to cover the skies over the northern hemisphere of Earth, and another in Chile to cover the skies over our southern hemisphere. The survey, dubbed Two Micron All-Sky Survey (2MASS), scanned the skies at particular bandwidths in the near infrared (IR) near 2 microns. The survey catalogued 300 million celestial objects, with an additional million specially cataloged with the "2MASS"-prefix. One of those objects was labeled, 2MASS J23062928-0502285; more commonly known as the star, TRAPPIST-1, so named because it was first discovered by the TRAPPIST robotic telescope situated high in the Andean mountains of Chile where the night skies are generally clear and dark.
TRAPPIST-1 was all over the news in 2015 when astronomers discovered it was host to two planets. Amazing as that was, things have become even more awe inspiring as scientists have announced the discovery of an additional five planets orbiting TRAPPIST-1!
TRAPPIST-1 is a very low mass ultra-cool red dwarf. As with all low-mass dwarf stars, TRAPPIST-1 experiences a low fusion rate. As such, the star exhibits a relatively-low energy output, equating to a lower average temperature than other main-sequence stars. As such, its planets would need have close, tight orbits if they are to enjoy what little radiant energy the star has to offer. As it turns out, they do as we'll discuss.
Main-sequence stars make up 90% of the true stars in the known universe; fusing hydrogen into helium at varying rates, with varying chemical compositions, and having varying masses (as well as other variables) between each other. When plotted on a graph in order of actual brightness (absolute magnitude) against their color indices, a band is realized as shown below:
The diagram above is called a Hertzsprung–Russell diagram. Since luminosity has a proportional relationship to absolute magnitude, these two measures can be plotted vertically. A star's spectral class is based off the star's color, therefore these two measures are plotted horizontally. Spectral classes include O, B, A, F, G, K, & M, with "O" stars emitting higher-energy blue, and "M" stars emitting lower-energy red (and into infrared). TRAPPIST-1 is an M-class star, therefore a low-energy red/IR emitter.
In our own Milky Way galaxy--at least in the neighborhood of our own Sun--M-class stars are the most common type of star (~76%). This fact alone is exciting, because it has been speculated for years that M-class stars could be the spectral type most likely to host life-bearing planets. There was an article in Scientific American from over 10 years ago discussing this potential in greater detail.
As such, the discovery of seven planets--three of them in the habitable zone--orbiting an M-class star in our own galactic neighborhood is a big deal. I'm not entirely sure why being Earth-sized is necessarily important; in fact I don't think it is. However, what is more important is that these planets are likely rocky (terrestrial). Further transmission spectroscopy studies will be conducted to confirm their terrestrial composition as we'll discuss shortly. A solid planetary surface provides liquid water places to pool, should water exist and under the right pressure and temperature circumstances. We'll get to these variables shortly too.
HOW TO FIND & INTERPRET EXOPLANET PROPERTIES (basic)
With the use of ground- and space-based telescopes, scientists are able to scan the skies for exoplanets. There are several ways scientists can detect exoplanets. One of the more common methods is to look for periodic dimming in a star's light. I'll keep this quick and simple as I'm aware this method is widely understood by everyone; media outlets have done a good job of including an explanation of this when reporting on the discovery of exoplanets. Also, there are many thousands of people voluntarily helping NASA detect planets on the NASA-funded website, www.planethunters.org
The method, called transit photometry, requires recording the light curve of a star's light (as well as IR which was the case with TRAPPIST-1). On a graph, with time on the x-axis, and visual brightness on the y-axis, a star's light would appear as a straight horizontal line. However, if another object passes between Earth and the star under observation, then it blocks a portion of that light. This causes the light curve to dip. Once the object has passed, the light curve returns to normal.
A dip doesn't necessarily equate to a planet. For this to be confirmed, the dip must recur. In fact, along the light curve, there must be a series of dips occurring at regular intervals along the x-axis (time). A predictable interval suggests a planet is orbiting the star. This all being considered after knowing some important properties of the star.
Just from this information, scientists can infer a considerable amount of information regarding a planet. The depth of an observed dip indicates the physical size of the planet. Deeper dips are caused by larger planets whose physical size blocks more starlight from reaching us. Smaller planets block less, therefore their dips are less pronounced.
The time interval between dips for a particular planet indicate its orbital period. Larger intervals (more time between dips) indicate longer orbital periods. From orbital periods we can infer distance a planet is from its star. This is done using Kepler's third law of planetary motion; the law of periods, itself arising from the universal law of gravitation, which itself correlates to the inverse square law.
So, from dips in a star's light curve, we are able to determine:
- The existence of a planet
- The diameter of said planet (physical size)
- The orbital period of said planet
- The distance of said planet from its star
Within the TRAPPIST-1 system, scientists have been able to infer one other piece of information not always possible from other planetary systems; mass.
5. The mass of said planet
Because the TRAPPIST-1 planets are so closely spaced, they gravitationally pull on each other such that their orbital periods are affected. The effect is subtle, but measurable with the right equipment. From this, scientists are able to infer the mass of each planet. From the universal law of gravitation, we know that larger masses exert stronger gravitational forces than objects of smaller mass; a direct proportion to mass. We also know that distance between objects affects these forces such that they are inversely proportional to the square of the distance between interacting masses (inverse square law).
Orbital periods of higher-mass planets will be less affected than the orbital periods of relatively lower-mass planets because the gravitational 'pull' from larger planets are greater than that of smaller planets in the system. Think of a tug-of-war between unevenly-matched participants. That may be a bad analogy. :/ I admit, I'm not a big fan of using analogies in science.
Based off the amount of distortion in orbital periods between planets (think density as it directly proportional to gravity), coupled with the planets' known sizes (think size in terms of volume), scientists are able to infer the masses of each planet. A known volume (size) with known density (gravitational force) can be multiplied to obtain mass.
Given this, careful measurements of the tugs between planets as observed on the light curve (distortions in time intervals between dips) can provide mass inferences for each planet.
All this from looking at the infrared light spectrum of TRAPPIST-1 using the Spitzer space telescope, a telescope whose initial purpose did not include finding exoplanets! But since this telescope is capable of observing the universe in the infrared portion of the electromagnetic spectrum, and the majority of electromagnetic energy emitted from TRAPPIST-1 is in the IR portion of the spectrum, Spitzer (with some ground-based tweaks) was the perfect candidate with which to observe the star.
I can't wait for the James Webb telescope to begin operations in 2018! That will be a future blog post!
I'm not going to get into details of the different planets found around TRAPPIST-1, because that information is published here for your perusal: http://www.trappist.one/#system
Above, I mentioned that in order to confirm the rocky nature of the TRAPPIST-1 planets, further transmission spectroscopy analyses would need to be conducted. Transmission, and its 'inverse', emission spectroscopy, are used to detect element types in a planet's atmosphere. We'll delve into this amazing technique shortly, and how it relates to absorption spectroscopy.
The two elements scientists look for are the lightest ones; hydrogen and helium. The detection of these elements suggest a gaseous planet. Gaseous planets tend to be composed primarily of hydrogen and helium like our own gas giants, Jupiter, and Saturn (Uranus & Neptune are considered ice giants).
Scientists did not detect hydrogen or helium in significant proportions on either of the first-discovered TRAPPIST-1 planets, so they're confident they are rocky. And though it may just be a formality at this point, they've yet to perform spectral analyses on the more recently-discovered planets; something that I'm sure is underway as I write this. Besides, hydrogen/helium gas planets are unlikely to exist below 1.5 Earth radii. All of TRAPPIST-1's planets are well below this hypothetical threshold.
Elements in a planet's atmosphere absorb electromagnetic energy from their host star at unique wavelengths. With the right equipment, we can look to see which wavelengths are being absorbed. This can largely be done within the visible light portion of the spectrum.
The portion of the electromagnetic spectrum most important in spectroscopy is its visible light ("visible" of course based off human eye capabilities). Based off what wavelengths are absorbed, or transmitted, we can determine which elements are at play.
If we look at an element's absorption spectrum, we'll see all but the light that element absorbs. The portions of the spectrum the element absorbs would show up as black lines along the otherwise continuous spectrum. If we look at an element's transmission spectrum, the opposite is seen; the wavelengths absorbed by the element will show up as black space (missing from the spectrum), leaving only the wavelengths that were not absorbed (reflected away or 'transmitted') to be seen. Below shows the comparison of the absorption and transmission (aka emission) spectrum of hydrogen.
Here are the absorption v. transmission spectra for other elements to help paint a better picture of what the science is all about:
By looking at the light reflected by assumed atmospheres of the TRAPPIST-1 planets, scientists will be able to determine their general makeup.
So let's look at some caveats regarding transmission spectroscopy of exoplanet atmospheres..
First, scientists aren't actually looking directly at an exoplanet's atmosphere. In fact, they don't even know if the density of an exoplanet's atmosphere is Earth-like, wispy-thin like Mars', thick and heaving like Venus', or barely traceable like Mercury's.
In order to "see" an exoplanet's atmosphere, scientists record the spectrum of a star while its planet is between it and us. This would give us the combined spectrum of both the star and its planet. Then, another spectrum is recorded when the planet is behind the star. This would give us the star's spectrum only. The difference between the two spectra reveals the planet's spectrum.
This method only works for planets with orbits very close to their host star. In many cases, this means being outside the habitable zone (too close to the star). However, since TRAPPIST-1 is such a low-mass, low-energy star, this closeness actually puts some of these planets within its habitable zone, and that's a big reason why scientists are so excited.
The circumstellar habitable zone, or habitable zone for short (aka the Goldilocks zone), is the zone around a star within which liquid water could exist on a planet's surface (if the planet has a solid surface).
A star's habitable zone is a function of its radiative flux, as well as its planets' masses and orbital radii. Higher-energy stars will have habitable zones further out, and vice versa.
When scientists study exoplanets, they are restricted in what they can actually see. Most of what they can find out about a planet is cleverly inferred. The methods they use differ depending on many factors. One major factor is an exoplanet's distance from its star, because this distance (relative to the star's brightness) affects the contrast of light we're able to detect between the planet and its star.
We can't see the planet until it passes in front of its star (transit). So being able to contrast it from the star is important. Without this ability, we would not be able to take the difference of the star and planet's spectra in order to determine a planet's atmospheric elemental composition; regardless of the atmosphere's unknown density.
For gas giants in large orbits distant from their host star, scientists can directly image the planet by blocking the star's light in order to focus on light from the planet. This of course means they'd be looking at planets far beyond the habitable zone. Any closer, and the contrast between the planet and its star would be lost.
For smaller Earth-sized and Earth-size'ish planets with smaller, closer orbits around their host star, scientists are able to use the transit method explained above. But usually, this means looking at planets inside the habitable zone. Inside as in outside of the habitable zone's inner edge.
So it has been, for the most part, that all the planets we are best able to infer information from, have been outside the habitable zone. The one's within it are notoriously difficult to observe for reasons relating to contrast being washed out. It's a conundrum to say the least. The planets that would be of most interest--the ones in the habitable zone--are the hardest to infer information from.
But that's not necessarily the case with the TRAPPIST-1 system.
Though I've not read this anywhere, it seems sensible that one of the major reasons scientists would be so excited about this system, is because the habitable zone of such a low-mass star is extremely close to the star itself.
As such, it would fit the criteria for conducting transit analysis on a rocky planet while actually being in the habitable zone, as opposed to being too close and orbiting outside the habitable zone's inner edge. This is a big deal in and of itself.
Do you remember a few years ago the incredible discovery of the exoplanet, Kepler-186F? That is an Earth-sized planet (well bigger than Earth actually) within its star's habitable zone, but its star is much brighter than TRAPPIST-1. Though both stars are red dwarfs, only TRAPPIST-1 is an ultra-cool red dwarf with energy emission low enough that most of its "light" is in the near-infrared portion of the electromagnetic spectrum.
As such, TRAPPIST-1's habitable zone is considerably closer than that of Kepler-186F, thereby making inferences from transit data more difficult for reasons listed above. This issue, coupled with Kepler-186F's greater distance from Earth (~500 light years away) makes for very difficult work.
Given TRAPPIST-1's closer distance (~39 light years away), and its planet's close orbits, scientists are likely to infer more than they have been able from Kepler-186F. Scientists still aren't sure if Kepler-186F is rocky, or a lower-density ocean planet, or some other variant.
But even with the advantages evident with the TRAPPIST-1 system, we still have to be aware that "habitable zones" do not equate to actually being habitable. In fact, they don't even come close to suggesting habitability. And as astrophysicist, Gabe Perez-Gil at the Columbia University's Center for Cosmology & Particle Physics, has said, the habitable zone isn't necessarily a prerequisite for habitability either. He has pointed out that rocky planets in large orbits beyond their star's habitable zone that could have thick GHG-rich atmospheres capable of pressures and temperatures conducive to liquid surface water. In fact, in consideration of "super-Earths" as NASA has called them, there are now two categories of habitable zones; the standard conservative type wherein low-mass planets like Earth could harbor water (haha, harbor water)... and a second, larger 'extended' zone within which large rocky "super-Earths" with strong 'greenhouse' effects can, heh heh, harbor water. Habitability, as Gabe put it, is more of a guideline than anything else.
With the power of Microsoft Paint, I give you these two modified images:
ARTIST RENDITIONS OF HABITABLE-ZONE PLANETS
To garner more interest, NASA employs the talents of artists who create stunning video and pictorial depictions of new exoplanets. I don't see anything wrong with this, and in fact is probably necessary as those they're trying to impress are often not necessarily educated in the hard sciences, yet vote for "leaders" who control NASA's paltry budget. Yet, it's important that we're aware these depictions are merely artist renditions and nothing more. They are by no means, representations of the planets they depict... or maybe they are. Point is, we simply don't know. We shouldn't let artist renditions replace the facts we do know about new discoveries, we should have those renditions inspire us to know more. The more we know, the better we'll vote (local, state, and national levels).
Assuming an atmosphere isn't too thick to see through, we can say with a degree of confidence that if one were able to sit on, say, TRAPPIST-1f, then it's reasonable to believe they would be able to see a sky punctuated by the glow of other planets. Not star-like specs such as what we see when we look up at Venus or Mars, but larger, more detailed views much like we see our own moon. Imagine that!
At any rate, imagination has a definite place in science; it really does. It has been the source of paradigm shifts, new discoveries, and new methods of observation for centuries. Without it, science would advance at a snail's pace, if at all.
But, that definite place in science is delineated by a very fine line between what is science, and what is not. It's a talent I believe we all can have, and one certainly possessed by the likes of Feynman, Einstein, and others. All we need is passion for what we learn, coupled with a solid scientific foundation upon which to exert our passion.
That's my two cents.
As always, thanks for reading.