Pale Terraqueous Globes

by Alexander Bastidas Fry

Image credit: NASA Ames/JPL-Caltech/Tim PyleImagine the closest star beyond the Sun has a planet orbiting it about the size of Earth. Visualize what your sunset would look like on this distant planet. Perhaps there would be two stars at the center of this solar system. Your sunset would be breathtaking. You could even visualize what the Sun would look like from this planet – just another unassuming star in the sky. You don't have to merely imagine that such a planet might exist. A planet like this really does exist – of course you'd still have to imagine the part where you are on the surface of this world. The Alpha Centauri star system, which is essentially a triple star system of Alpha Centauri A, Alpha Centauri B, and Proxima Centauri has just such a planet. There is a planet in the sky waiting for us at a distance that is just two hundred and seventy thousand times further than the Earth is from the Sun. This planet is near 1500 degrees on the surface, so we wouldn't want to be there, but nonetheless the fact is that astronomers are finding similar planets commonly. There may be a planet just the size of Earth at a nice temperature quite near us galactic speaking. We are searching.

Most planets don't seem to be much like Earth. In fact so far we haven't found a single planet that has a temperature and size similar to Earth, but part of the problem with finding planets is that finding big giant planets – like Jupiter is easy – while small rocky planets like Earth are elusive. But we are on the edge of discovery. All in all Earth-like planets likely abound. In fact with 95% confidence there is an Earth size planet in the habitable zone of a small star within 23 light years of us. The habitable zone is the place where a planet would not be too hot or too cold. A place where a planet wouldn't see its oceans boiled off or frozen into desolate ice tundra. Habitable planets are common in our galaxy and by galactic standards not very far apart. On average Earth-like planets are only 13 light-years apart.

Just a few years ago we knew very little about the characteristics or numbers of planets beyond our solar system—the unknown extrasolar planets. Today we know that most stars are host to at least one planet. This is revolutionary: not only do other stars with other planets exist, they are downright common. This new information was harvested by the Kepler space telescope. It systematically surveyed 145,000 stars in the direction of constellation Cygnus for the past four years. This careful survey allows us statistically extrapolate the occurrence of planets for each of the hundreds of billions of other stars in the Milky Way. We have observed on average that each star in the Milky Way has more than one planet. There are several ways to detect or infer the presence of extrasolar planets. The most common and useful methods to date for detecting stars are the radial velocity detection and transit detection.

The radial velocity method of detecting stars relies upon Newton's Third Law of motion: every force has an equal and opposite force. So as the Earth, or any planet, swings around a star the star also swings around the given planet. If that movement is radial (parallel) with our line of sight then we can observe the precise variation in wavelength of light emitted by the star (the Doppler shift) over time to infer the existence of a massive object, like a planet, orbiting that star. Stars moving towards or away from us at speeds of as little as 1 meter per second can be found using the radial velocity technique.

The transit method of planet detection is looking for minor eclipses. We detect planets by watching them eclipse their host star, or transiting across the face of the host star. Such eclipses are far from total and are exceptionally hard to notice. This is the method that the Kepler telescope utilizes. If a star's light dims or brightness we make take notice, but it could be a stellar flare, a binary star companion, noise in the data, or a myriad of other effects. But if the star dims by the same amount over a repeated period then we can take this evidence to deduce a planet may be orbiting that star.

On February 26, 2014 the Kepler Satellite science team revealed observations of 715 new planets based on data that had been taken over the last four years. Currently the Kepler satellite is in a bit of a bind. The reaction wheels that allow it to precisely orient itself in space have failed. So these new planets coming in are just analysis of data on hand and there is no fix for Kepler in sight. The next frontier in extrasolar planet detection relies upon a technique that has not been possible before: astrometry or the precise measurement of the position of stars. Stars move when tugged upon by planets that may be orbiting it (this is the same as in radial velocity method, but here the movement is in the plane of the sky). Less than a month ago the Gaia space telescope settled into its orbit where it will soon begin to observe and pinpoint the position of nearly a billion stars. We don't expect that most of these stars will have a detectable planet (yes they may have planets, but not detectably so), but we expect to find some strange worlds for sure.

When we think about other planets we might have to change our expectations somewhat. Most stars are slightly less massive and cooler than our sun; thus for the planet to be of the same surface temperature as that of Earth the planet would need to orbit closer. It would be a shorter year. And of course unlike science fiction it isn't likely a planet would have an atmosphere that would be comfortable to Earth accustomed organisms. The Earth's atmosphere is generated by the collective activity of trees and green algae in the ocean. Ultimately researchers have urged us to consider that planets are so unique that habitability should be evaluated on a case by case basis. In fact, even the Earth's long-term habitability is in question. The Earth exists precariously close the inner edge of the habitable zone, where it might one day be too warm for comfort. In fact in billions of years the Sun will most certainly heat up and expand to the point that Earth will be a poor place to look for life.

Philosophically it feels as if the existence of other Earth-like planets is monumental. Yet given the distances to these objects it is hard to fathom any tangible consequence for generations to come. François-Marie Arouet, better known by his pen name Voltaire, was a natural philosopher who was one of the first people to consider with objective reason what other planets would perhaps be like. Voltaire's style was a mix of story and inquiry as was common at the time. One particular short story Voltaire wrote alluded to the myriad of planets that he speculated about may exist. The story is Memnon, the Philosopher of Human Wisdom Memnon, the Philosopher of Human Wisdom. It tells the story Memnon who decides to become a philosopher one day and upon that same day he loses his eye, his health, his fortune, and his reason. He passes into sleep in despair at the end of the day and is visited by a celestial spirit in a dream. The spirit says that things could be worse, in fact the spirit states that there are a hundred thousand million worlds and in each world there are degrees of philosophy and enjoyment, but each world has less than the next; there is a world of perfect philosophy and enjoyment somewhere the spirit implies, “There is a world indeed where all [perfection] is possible; but, in the hundred thousand millions of worlds dispersed over the regions of space, everything goes on by degrees. There is less philosophy, and less enjoyment on the second than in the first, less in the third than in the second, and so forth till the last in the scale, where all are completely fools.” Memnon is afraid that the Earth must be on the low end of the list and replies, “our little terraqueous globe here is the madhouse of those hundred thousand millions of worlds,” a statement which predates and echoes Carl Sagan's sentiments on our pale blue dot.

Voltaire seems to have identified something fundamental about the existence of other planets: there presence is not enough. For example most rocky planets we find lack any atmosphere we would find acceptable. There is no reason to think that other planets have atmospheres that humans could survive in, in fact oxygen is in a non-equilibrium state on most planets due to surface geological activity. Even if a planet has an atmosphere it may quickly fade unless the right conditions on the planet are maintained. Our oxygen rich atmosphere was primordially generated by the collective action of Cyanobacteria in the oceans some 2.4 billion years ago. This great oxygenation event effectively poisoned previous incarnations of life on the planet, but gave rise to rapidly respiring, and thinking, creatures we know today. Perhaps cyanobacteria could be seeded into the oceans of barren planets in the habitable zone and in time the atmosphere of that planet would have enough oxygen for us to breathe easy. Or maybe we could find a planet with oxygen in the atmosphere already. If astronomers ever detect the spectral signature of oxygen on an exoplanet we could optimistically infer there are respiring plants, and maybe even creatures, on the planet. Such a detection would be carried out in a similar way in which we detect the spectral signature of elements in distant stars, but because planets are so dim it may take a truly monumental telescope of perhaps one hundred meters in diameter to achieve such a detection. Even if there was an atmosphere there are still issues of temperature, seasonal variation, geological activity, natural resources, weather, and perhaps even conflict with the natural residents of the planet. The moral implications of visiting a thriving planet give rise to comparisons to colonization.

We don't have to pretend there are planets beyond Earth to visit anymore. We live in a universe, or at least a galaxy, that has given us an embarrassment of richness in planetary diversity, however there is no guarantee that any of the planets beyond Earth are better than Earth.

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