How Do Alien Worlds Reveal Themselves?

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Astronomers can detect the presence of exoplanets by measuring the dip in light from stars, gravitational wobble and even through microlensing events.
ESO

Exoplanets are hard to find, but astronomers are developing increasingly ingenious ways of detecting their presence.

You might think that finding a planet orbiting a distant star would simply require a bigger telescope -- after all, bigger telescopes reveal fainter and finer details. So why not build a really, really big 'scope and you can chalk up dozens of exoplanetary discoveries! Right? Not quite.

Detecting these alien worlds is made difficult because of their close proximity to bright stars, making them incredibly challenging to see directly. Also, if the exoplanet is located far enough away from its host star, the meager light it reflects would be too faint for even the biggest optical telescope. It is for these reasons that increasingly ingenious indirect techniques of detection are being relied upon. Only 7 percent of exoplanets discovered so far have been observed directly.

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Exoplanet Wobble

Distant stars will exert a gravitational pull on their planets -- thereby keeping them in orbit -- but at the planet's gravity will also tug on the star.

Instead of the planet orbiting the star, the two celestial bodies orbit a point where the forces of gravity equal. This will lie at a point close to the center of the star depending on the mass of the two objects.

The star will move very slightly off its center of mass -- a point known as the "barycenter" -- much like a hammer thrower spinning around before releasing the hammer. By studying the light emitted from the star, we can see its slight movement by detecting a shift in position of spectral lines -- measuring this shift enables us to determine the approximate mass of the planet. This is known as the Radial Velocity Method.

Astrometry is an extension of this method, where the gravitational wobble is big enough for telescopes to see the star shift in position as an exoplanet orbits. However, the shift in position is usually tiny, making this method problematic.

In the case of super-Earth HD40307g, the exoplanet was discovered by Europe's High Accuracy Radial velocity Planet Searcher (HARPS) instrument, a spectrograph installed on Europe's La Silla Observatory in Chile that can sense the very slight shift in the star's spectrum, thus revealing the presence of a candidate exoplanet in the star's habitable zone.

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In some exotic cases, a similar method is used to infer the presence of planets around pulsars -- rapidly rotating, extremely dense neutron stars. As they spin, an intense beam of radiation sweeps around with it, much like the beam of light from a lighthouse. If Earth lies in the line of sight of the beam, a pulse of energy is seen. These pulses are incredibly accurate and give rise to the name of the type of star.

The presence of a planet in an orbit around the pulsar causes it to wobble a little due to the gravitational pull of a planet and this can be seen to affect the otherwise accurate timing of the pulses. By measuring the variability of the pulse, its possible to determine the orbital characteristic and mass of the planet.

When an Exoplanet Transits

In other cases, the orbit of an exoplanet is aligned perfectly so it appears edge-on when viewed from our vantage point. This gives us the rare opportunity of observing it as it transits across the front of its parent star. By measuring the dip in starlight, it is possible to determine the physical size of the planet, providing an insight to its physical properties. This detection technique is known as the Transit Method.

NASA's Kepler space telescope was designed to detect the very slight "dip" in starlight as an exoplanet transits its parent star. So far, the mission has identified over 2,300 exoplanetary candidates (i.e. signals that still need to be verified that they are actually exoplanets) in a small viewing area of the Milky Way.

In multi-planetary systems Transit Timing Variations (TTVs) may be detected in transiting exoplanets. Slight variations in their orbital timing can reveal the gravitational presence of a nearby world, even though it may not be visible.

In a handful of cases, Gravitational Microlensing is caused when a star passes in front of another, more distant star. The gravitational field of the closer star causes the light of the distant star to "bend" around it, much like a magnifying lens. A spike in brightness will be detected. Should there be any exoplanets orbiting the closer star, their gravitational presence will leave a signal imprinted in the microlensed light.

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There are other more refined techniques, but for me the real excitement will come when we can finally directly observe an exoplanet's atmosphere and surface detail.

Until then, we will have to leave it to artists and science fiction writers to give us a glimpse as to what these alien worlds look like. But with the way technology is advancing, I don't think we will be waiting for too long for our first exoplanetary photo opportunity.