Exoplanets survey

 kw: book reviews, nonfiction, astronomy, astrophysics, exoplanets, surveys

Imagine what planetary astronomy would be like if there were frequently two or even three planets visible in the sky that appeared similar in size to the Moon as seen from Earth. And I do mean "frequently." The planet currently known as Trappist-1e, the fourth planet around the small star Trappist-1, has a six-day year. From a dark-sky location, on the side away from its sun, the next planet outward (-1f) would reach superior conjunction about every dozen days, having a visible size of 34 arc-minutes; we see the Moon as about 33 arc-minutes across. The apparent size of -1g, another step outward, ranges up to 19.5 arc-minutes, nearly 2/3 of a "moon unit". From locations near the terminator (the star-rise or star-set line), the next planet inward, -1d, approaches 32 arc-minutes at inferior conjunction, as a crescent, just as Venus at its brightest is seen as a crescent because it is nearly between the Sun and the Earth. Even when the other six planets are at opposition (on the other side of the star), they always appear larger than any of the planets in our solar system ever do: even Venus at near-conjunction is too small to show a visible disk, except to a few folks with test pilot vision. All the Trappist-1 planets are always seen as disks.

From planet -1e, however (and any of the others, from -1b through -1h) there probably is no star-rise or star-set. So far as we can tell, they are all in tidal lock with Trappist-1, the way the Moon is with the Earth. It is also unlikely that three big neighboring planets can be seen in the sky at the same time because the orbital periods are all in resonance, which keeps them from lining up in the sky simultaneously.

If you were on the brighter side of the planet, the star would appear much larger than the Sun, about seven times its size. But its surface brightness is lower, by far, than the Sun's. It would still be risky to look right at it. Imagine looking at a ball of near-molten tungsten at a temperature of about 2,560K (~2,290°C or ~4,150°F), just slightly cooler than the filament in an incandescent light bulb. Although astronomers call this star a "red dwarf", one of the reddest known, visibly it appears orange-white. It would be dazzling, even though our Sun is 1,800 times as bright.

All of these facts are consequences of the small size of the Trappist-1 stellar system. The outermost known planet, Trappist-1h, revolves at a distance of only 8.8 million km from the star (from Earth to the Sun is 149 million km). The star's faintness, however, means that the habitable zone is between 3 million and 5 million km. Both Trappist-1d and -1e are within this zone, and maybe -1f and -1g, while -1h is "out in the cold" and always frozen (like Mars) and -1b is rather like Mercury and subject to searing heat (from 125°C to 230°C, or 255°F to 505°F).

All of this has been learned from viewing the Trappist-1 stellar system using several telescopes (at least 3 of them in space) and spectroscopes. The procedures and the kinds of data needed to discover and characterize exoplanets are described in a very understandable way by Joshua Winn in The Little Book of Exoplanets. Considering that there are more than 5,500 known exoplanets presently known, Dr. Winn makes a good point that we need to set aside the term "exoplanet" and just call them "planets." Our stellar system, which includes eight planets, is just one of more than 4,100. More planets, and more systems having two or more confirmed planets, are being discovered daily.

All of the earliest discoveries were made using the Doppler method, which measures how much the star is moved back-and-forth by a planet. Naturally, the easiest sort of planet to discover, by nearly any means, is one that is large (that is, massive) and close to its star. Big Doppler shifts are easier to discern, and shorter orbits take less time to confirm (days or months rather than years or decades). For one method, though, direct observation using a coronagraph, big planets that are far from the star are easiest to see. Finding smaller, less massive planets, in years-to-decades-long orbits is between difficult and (so far) impossible. None of our current methods could reliably discover Mercury, Venus, Earth, or Mars, and also Uranus and Neptune. Saturn and Jupiter are on the "possible edge". So it is no surprise that systems similar to our own have not yet been observed. Proclamations that our system represents something very rare are premature. We don't yet have a way to know!

The most prolific method so far is the transit method. When the system is lined up just right, one or more of its planets will periodically cross in front of the star, which dims its light a tiny bit. This illustration compares observations of a transiting planet made from Earth's surface with observations made using the Kepler Space Telescope. The atmosphere, even using adaptive optics, is a big handicap to precise observation.

During the useful lifetime of the Kepler telescope I used Zooniverse to make some of the measurements in a Citizen Science project called Planet Hunters. I don't think I made any new discoveries, but I think my work helped others confirm at least a few of them; the stats show that I made 6,866 classifications. Most of them were "no planet".

This image, Plate 16 in the book, shows the Starshade concept: A space telescope will have a co-orbiting daisy-shaped shade that blocks the light of one particular star with sufficient efficiency that direct observations of planets near the star can be made. The shape of the bladed disk is optimized to "spread around" diffraction effects so that a star's light can be reduced by a factor of several million or even a billion, while the nearby space, within a fraction of an arc-second, is unimpaired.

Such a system could see Earth, probably Venus, and maybe Mercury. "Co-orbiting" in this case means the telescope and the shade are about 30,000 km apart in a very high orbit! They would need to be supplied with large amounts of fuel to allow for frequent re-positioning and re-aiming. Perhaps by the time these are commissioned, paid for, built and orbited, we'll finally have a successor to the Space Shuttle that can reach high orbits so they can be refueled. Running out of fuel is the bugaboo of space observatories.

This is more than just curiosity. Every space observatory since Hubble has had as part of its ambit, "Help find habitable planets and planets with life, even intelligent, communicating life". Whether or not Earth is utterly unique in the Galaxy (if not the whole universe) is the biggest question science can address. (My note: It's curious that the budget of NASA is in the range of 1/200th of the Federal budget. The yearly spend of SpaceX and all the other private rocket companies that have sprung up in the past decade or so totals between 1/10 and 1/5 of NASA's budget. To get a "supershuttle" into operation and to genuinely support "big space astronomy", these numbers have to increase by a factor of ten or more.)

I am quite enamored of exoplanetary science. This "little book" is packed with great info and stories about its current condition.