Planets of Other Stars

NASA – NASA’S Kepler Mission Discovers Its First Rocky Planet. It is a little bigger than the Earth, but it orbits its star every 20 hours. This makes its temperature something like 1800 K / 3300 F, as hot as a lightbulb filament in action.

This is only the latest in a remarkable revolution in astronomy: the discovery of extrasolar planets. Since the 1990’s, over 400 of them have been discovered, and some of them have very startling properties. Several of them are “hot Jupiters”, gas-giant planets that are unexpectedly close to their primary stars. They most likely formed much farther out, then spiraled in. Even more oddly, many of them have very eccentric orbits, with eccentricities typically around 0.4 and as much as 0.7 — 2 to 6 times farther at farthest distance than at closest distance. Near-collisions between inspiraling planets? (More on this mystery) If they get close enough, their outer layers may evaporate off into outer space, leaving a rocky core, like the planet I mentioned above.

There are even planets orbiting pulsars, of all places. How did they get there?

To date, all the planets discovered orbiting “normal” stars are at least a few times more massive than the Earth, but that’s a side effect of detection sensitivity. The smaller the planet, the smaller the effect. But as astronomers improve their techniques, they may eventually detect Earth-mass planets.

Despite that, astronomers have often been able to detect more than one planet orbiting a star, and in the case of Gliese 581, at least 4 and possibly 6. Some of Gliese 581’s planets orbit at about the right distance to have liquid water on their surfaces, thus making them potentially habitable. However, Gliese 581 is a red dwarf, a relatively faint star, meaning that these planets orbit close enough to become tidally locked. So Gliesians, if any, would see their sun more-or-less fixed in the sky.

Some history and detection details:

Early speculations and binary stars

Over the centuries, some people have speculated that there are other Earthlike worlds, About 2400 years ago, Democritus of Abdera speculated that there are many worlds, some with more than one Sun and Moon, and some with none (each world is roughly a Ptolemaic cosmology). Epicureans thought likewise, but that idea went out of style until early modern times. God would have told us about other worlds if they existed, some theologians may have thought.

But with the rise of Copernicanism in early modern times, some people started speculating that other stars have planets. I’ve been unable to find out what arguments they had had, but they may have argued that God had made the Sun luminous to light its planets, and that God would not have wanted other stars’ light to go to waste. That sort of argument seems very silly, but in the 18th century, some people argued that every planet must be inhabited because God would not waste a world.

But in the 19th century, however, astronomers found better reasons for suspecting that other stars have planets. They observed that numerous stars are very close to other stars, and by repeating their observations over the years, they discovered that those stars were orbiting each other. This is not, of course, direct evidence of planets, but it is evidence that stars often have other objects moving around them. If there are large objects, there are usually also lots of smaller objects, suggesting that many stars have planets.

Starting near the end of that century, they were using an additional technique: the spectroscope. It sorts light out by wavelengths, making spectral lines apparent. One can then find out how much a star’s light is being blueshifted or redshifted. That led to the discovery of spectroscopic binaries, stars which cannot be resolved but which have overlapping spectra. As the stars orbit each other, their spectra get alternately blueshifted and redshifted from their average.

Still another type is the eclipsing binary, where the two stars eclipse each other as they orbit each other. Algol (Beta Persei, “The Ghoul”) is the best-known one and also one of the first spectroscopic binaries discovered (Hermann Carl Vogel, 1889).

There are even binary pulsars. As a pulsar orbits its companion, its pulses take alternately more and less time to reach the Earth. Joseph Hooton Taylor, Jr. and Russell Hulse discovered the first known one, PSR B1913+16, in 1974, and since then, several others have been discovered.

In many binary stars, it is only possible to observe only one of the companions. Friedrich Bessel discovered that about Sirius and Procyon in 1844, though those stars’ companions were observed some decades later. Likewise, many spectroscopic binaries are one-line ones, with the companions too faint to be noticeable. Many binary pulsars also have unobserved companions.

As we shall see, astronomers have used all these techniques to search for planets.

Planets Observed

Since the early 19th century, several astronomers have claimed to have discovered extrasolar planets, all by studying the changing positions of various stars (astrometry). However, most of these purported discoveries are now discredited.

But the first substellar objects observed orbiting stars were disks of dust and gases. These were discovered in the 1970’s and 1980’s by observing in the infrared, where the disks would glow, and where their stars would not glow very much. These stars were very young, and protostars were observed surrounded by dust and gases, so one concludes that these disks are leftover material from the stars’ formation.

This was even more suggestive evidence, since unless the Sun was some cosmic freak, it also had had a disk of material orbiting it in its first few million years. That disk would have formed its planets, and that process would have happened elsewhere also.

Astronomers kept looking for planets, and Aleksander Wolszczan and Dale Frail made the first unambiguous discovery of extrasolar planets for the pulsar PSR B1257+12 in 1992, using pulsar timing. Three years later, Michel Mayor and Didier Queloz discovered a planet of a more ordinary star, 51 Pegasi, using a very high-resolution spectroscope.

Since then, astronomers have discovered many more exoplanets (short for extrasolar planets).

Many of them have been detected with high-resolution spectroscopes, which can have resolutions as good a 1 m/s. That’s 3.6 km/h or 2.2 mph — typical walking speed. That’s more than enough to detect Jupiter (28 m/s), but it’s not enough to detect the Earth (0.09 m/s). More recently, several exoplanets have been discovered by looking for transits, where the planet eclipses its star as it orbits. Jupiter would block out 1% of the Sun’s light, Earth 0.01%. Some satellites are now in orbit for doing this, notably the CoRoT and the Kepler satellites.

Some have been detected with both techniques, and doing so gives us both their masses and their sizes. This allows us to estimate both the mass and the size of each planet, and thus each planet’s density. Thus, in turn, provides clues as to its composition. A planet of star GJ 1214 has a mass about 6.6 times that of Earth and radius 2.7 that of Earth, giving an average density only 1.9 that of water. So it is either (1) largely ocean, (2) with a thick atmosphere of H and He, or (3) both.

A few exoplanets have been observed directly, notably one of Fomalhaut. It has a thick ring of dust around it, and just inside that ring is a planet that likely has a dust ring of its own. That planet orbits at over 100 times the Earth-Sun distance, with a period of nearly 900 years.

Resources
Extrasolar planet – Wikipedia, the free encyclopedia
List of extrasolar planets – Wikipedia, the free encyclopedia
Methods of detecting extrasolar planets – Wikipedia, the free encyclopedia
The Extrasolar Planets Encyclopaedia
Exoplanet Orbit Database | Exoplanet Data Explorer

Artistry:
7 Amazing Exoplanets [Interactive]: Scientific American
Hunt for Alien Earths: Extrasolar Planet Art Gallery
Extrasolar I
Extrasolar Planets – Space Art and Astronomical Illustrations

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