Chapter 9: Planet Formation and Exoplanets

Direct image of exoplanets around the star HR8799 using a vortex coronograph on a 1.5m portion of the Hale telescope

Image of a planet transiting a star, for illustration of transit method of discovery of extrasolar planets.

Multiwavelength Secondary Eclipse of Exoplanet GJ 436b

HD 80606b Infrared Light Curve

One of the long-time fantasies of many scientists and science fiction writers has been the direct imaging of a planet orbiting an alien star. When astronomers say "direct detection" or "direct imaging" they mean that they have collected reflected light from the planet itself rather than detecting its presence from observations of the star. This is an extremely difficult task.

The characteristic of a telescope that most determines its ability to find planets is the diameter D. The diameter or aperture determines both the telescope's sensitivity — that is, how much light it can collect — and its resolution — that is, how much fine detail is in the image formed by the telescope. Directly detecting light from a planet means distinguishing an extremely faint object very close to a much brighter one. Consider first the problem of detecting a planet that is reflecting light from its host star. The planet can only reflect the amount of light that actually hits its surface. The Earth's radius is 6370 km. While that's a large number by human standards, consider that the Earth orbits the Sun at a distance of 1 A.U. = 149,600,000 km. At that distance, the light from the Sun has spread out to fill a sphere with a surface area 4/3π × 149,600,000² square kilometers, whereas the illuminated surface area of the Earth is just ⅔π × 6370² square kilometers. The ratio of these two surface areas sets the upper limit on the Earth's brightness. So the fraction of the Sun's light that the Earth can reflect is — at most — only 2×10^-9. That's really faint, nearly a billion times fainter than the Sun.

When planets are younger, we can take advantage of the fact that they are still hot after their formation. The heat is generated as the planets contract under their own gravity and we can detect this heat as infrared radiation. Young planets radiating their heat of formation can be more than a thousand times brighter than they would be in reflected light, and so can be detected further from the star.

Painting of the Milky Way galaxy, showing the area that the Kepler mission is searching for extrasolar planets.

Stars are a long way away from us. The closest Sun-like stars Alpha Centauri A and B (two components of a triple system, along with Proxima Centauri) are 4.4 light-years away. That's roughly 40 trillion kilometers. To find out how far away from the star a planet at 1 A.U. would appear to us (its angular separation), we can calculate it as arcsin(149,600,000 / 40 × 10¹²) = 0.0002 degrees. Astronomers frequently measure such small angles in arcseconds. An arcsecond is 1/3600th of a degree, so 0.0002 degrees = 0.8 arcseconds. For comparison, if you hold out a single strand of hair at arm's length it is about 1 arcsecond across — a very small angle. Most stars are even farther away which means their planets will appear to be even closer on the sky.

The problem is made even harder by the Earth's atmosphere. The biggest astronomical telescopes (8 to 10 meters in diameter) could easily detect objects as bright as a planet was by itself in the sky. Such large telescopes also have the theoretical resolving power to see a planet as close to its star as 0.1 arcseconds. The problem is turbulence in our own atmosphere. Turbulence in the air above a telescope corrupts the incoming starlight — this is what makes stars twinkle. This twinkling spreads the light of a star out hiding the faint planet signal and making it so that even the biggest telescopes in the world have only about 0.5-arcsecond resolution. One possible solution to this problem is a technique called adaptive optics (AO). With adaptive optics, astronomers measure the turbulence in the atmosphere above their telescopes, then correct it by bending a flexible secondary mirror quickly enough to take out the distortions. This technique has its limits, though. Another solution is to send telescopes into space. This is more expensive and limits the size of the telescope. Even in space, detecting a planet that's a billion times fainter than a star at such small separations is difficult.

The first exoplanet to be discovered by direct imaging was seen in 2004. As a sign of the difficulty of this method, over 150 exoplanets had already been found by the radial velocity or transit methods. The contrast was favorable because the star was a brown dwarf — a dim and low-mass type of star not hot enough to fuse hydrogen into helium. Brown dwarfs are often and more appropriately referred to as failed stars. The companion is estimated to be three to ten times the mass of Jupiter, and it’s young and still glowing red-hot. Over the next few billion years, it will cool and shrink, ending up slightly smaller than Jupiter.

Despite these challenges, fifty exoplanets have been imaged. That’s a tiny fraction of the over five thousand that have been discovered, but it’s still impressive. Unsurprisingly, most are large—five to ten times the mass of Jupiter—and much farther from their stars than Jupiter is from the Sun. Hot Jupiters are not present in our solar system, and giant cold Jupiters, as some of these are, are unfamiliar too. The most impressive direct image is of a system with four massive planets, between six and nine times the mass of Jupiter, orbiting HR 8799 — a star in the constellation Pegasus that’s bright enough to see with the naked eye. The star is thirty million years old, so these planets formed relatively recently. They’re bracketed inside and outside by a disk of cold gas and dust left over from their formation. The lightest planet ever imaged is a super-Earth that orbits Proxima Centauri, a red dwarf star only 4.2 light-years away. Proxima Centauri, in the constellation Centaur, is the closest star apart from our own Sun. This planet is tantalizingly close, but unfortunately, so frigid that it’s unlikely to be habitable.

Interferometry is one of the tricks that can improve the results of direct imaging and astrometry. Combining the light from several telescopes results in much better resolution — the resolution is equivalent to that of a single telescope the size of the distance between the telescopes. So instead of building bigger telescopes scientists can design several smaller telescopes that are widely spaced apart and get the same improvement in their images. This technique has been used in radio astronomy for the past 50 years (for example, the Very Large Array in New Mexico) but it's a relatively new technique in optical astronomy. Nulling is a type of interferometry and is a way of suppressing a star's light. In this technique, two images are taken of a star. They are combined such that the two images are offset by exactly half a wavelength. This cancels out the light from the star but not from nearby objects that are not exactly offset. Thus the dim light from a planet orbiting the star is revealed and enhanced.

Other tricks are helping scientists see dim planets next to the bright stars they orbit. Coronagraphs are a way of physically blocking out the light from a star. Originally designed in the 1930s to block out the Sun's light in order to study its corona, they are now being used to block out the light of distant stars in order to detect surrounding planets. There are still problems with light refracting around coronagraphs and interfering with the faint light from planets. Astronomers are experimenting with differently shaped coronagraphs and different types of apertures in their telescopes to eliminate this problem. Exoplanet detection is a great example of a scientific field where progress is driven by technology.