February 25, 2017

Seven Distant Worlds, With Dramatic Implications

NASA has announced the discovery of a solar system 40 light years away that contains seven planets, each roughly the size of Earth. Three of these planets orbit at a distance that is considered habitable.

While some who read this are excited about all the discoveries for future generations, others dismiss it as something invisible to our generation and therefore useless. The fact is that it will be quite visible quite soon, and may be a major driver for technology advancements for the rest of our lives.

Understanding the Trappist-1 System

This is a small solar system around a small, cool star. Each of the seven planets is roughly the same size as earth. The smallest is roughly half-way between the size of Earth and Mars. All these planets orbit very close to the dim red star. It is assumed that, like the Earth’s moon, they will be tidally locked. The same side will face the sun constantly. In a system such as this, it is assumed that the dark side will remain relatively cool on the hottest worlds, and the permanently-sunlit side will be tolerably warm on the coldest world. Since all the planets are nearly earth sized, they should all have weather. This weather should circulate temperatures across the surfaces of each world. In other words, all seven could theoretically support life.

We know something about a small, tidally locked system like this from the moons of Jupiter. The four largest moons are between the sizes of our moon and Mercury. They orbit much closer to Jupiter than the Trappist system does to their sun. We see another factor in this situation with the Jovian moons, however, which may come into play here. The four moons are heated by internal tides, which squeeze and stretch them with each orbit. This generates heat. So despite being in a place in the solar system that would freeze most water worlds solid, both Europa and Ganymede (the two middle moons) have sub-surface oceans of liquid water. The innermost moon, Io, is basically a ball of volcanic sulfur that is so heated that it’s volcanoes turn the moon inside out repeatedly through history.

The TRAPPIST-1 sun is cool, but apparently young – no younger than 500 million years, or a ninth the age of our solar system. It’s difficult to say how old it is, but stars this size could exist for not billions, but twelve TRILLION years. If life does get a foothold here, it has a LOT of time to grow and explore before the local sun explodes or goes dark. Of the three planets most likely to be habitable, two are 2/3 the size of Earth (Mars is 1/3), and one is 1.34 times the size of Earth. The margin of error on the heavy one is such that it may be Earth sized. We aren’t sure of the size of the outer planet, and there may yet be more planets farther out in orbit.

Near Term Implications

The team responsible for building the James Webb Space Telescope (JWST) and the European Extremely Large Telescope (EELT) have stated that they may be able to measure the composition of the atmospheres of these worlds. They may be able to detect ozone, methane, or other gasses that indicate life, or the lack thereof.

When you consider this news, along with similar news about a super-Earth orbiting the nearest star, what does that mean in our lifetimes? Quite a bit.

It basically means that we have a very good reason to develop large telescopes specifically to examine near-solar exoplanets. Part of the reason we don’t have these yet is that they require either very large mirrors, or a cluster of telescopes flying in formation with incredible precision. Even being off the perfect distance by the width of an atom would ruin the observations. These projects could also be very expensive, exceeding $10 billion. That was not be a defensible investment if near-solar exoplanets were not both common and interesting. Now we know they are both.

There is a plan to build a star-shade that would block the light of a star, allowing the telescope to see the planets. The New Worlds Mission would build either a 4 meter telescope and starshade for $3 billion, or a starshade for the JWST for $750 million.

Some systems, such as those involving telescopes or star-shades flying in long range formations, have limited practicality due to how difficult they would be to re-orient to new targets. It becomes more practical in this case as well, since TRAPPIST-1 could be the focus for years of observation while still returning useful data.

FOCAL - Proto-Starship

Speaking of precisely-aligned, one shot missions, we have yet another option for examining these systems. As it turns out, you can go far enough away from our sun to use the gravity of the sun itself as a lens to curve distant light around to a focus – like a gigantic refractor telescope. To do this, one must send the “eyepiece” and camera of this telescope in the exact opposite direction as the star being observed. This could see not only the planets, but continents on the planets over a period of several years. As the probe gets farther from our sun, the magnification of the telescope would increase with the focal length, although the brightness would diminish. Ultimately, the images became too dim to resolve, and the field of view would become too narrow.

I could see one, or possibly several, telescopes sent out in this way to resolve the sun and planets from slightly different fields of view. Aiming straight at the star would give a bright target for the later observations, but would be somewhat useless for seeing the planets, which would always be in transit. It would be good for observing their atmospheres, however. The lateral telescopes could see the planets in crescent and gibbous phases as they went by.

The other beauty of the FOCAL mission is that it builds on our existing ability to send spacecraft beyond Pluto, but does so at a range that is possible with existing technology. Pluto is 40 astronomical units from the sun, or 40 times the distance from the Earth to the Sun. To work, FOCAL must go 550 astronomical units away or greater. So basically, take New Horizons, and give it a nuclear power supply that would still work in 220 years. Obviously, if you have 220 years to build a faster vehicle, you have better options. That said, 550 AU is still 0.009 light years, or less than one-thousandth of one light year. We must walk before we can run. Sending a vehicle one-thousandth of a light year is far easier than sending one four light years to the nearest star, or forty light years to TRAPPIST-1. Building something that can go 550 AU in less than a decade or two would be a key intermediate step before building an actual starship. The reward would be seeing these worlds in some detail, while building the next major set of technologies to building starships themselves. Eventually, a new generation of nuclear or electric engines could drive the FOCAL mission into the icy Oort Cloud. Visiting this space, with its many comets and possible ice worlds of planetary size, is a worthy goal in its own right.

Technology Spin-Offs

Any of these massive telescopes will drive the state of the art in precision manufacturing, optics, and imaging systems. We will see the atmospheres of these worlds in three years. We will develop very advanced telescopes to see many beautiful mysteries of deep space in the decades that follow. And we will benefit from technologies that expand our ability to gather information about nature at every level.

Thank you, TRAPPIST-1. For a star too dim to see with the unaided eye, you will light things up dramatically for our generation and those that follow. If our descendants live peacefully on these worlds ten trillion years from now, we will have bragging rights on being the generation who found them a nice stable home.

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