1 Hellish exoplanet has Earth-like density and composition on Fri Nov 22, 2013 11:46 pm
An artist's depiction of Kepler 10b, another planet that is tidally locked and orbits close to its host star.
The hunt for Earth-like planets orbiting other stars is inherently challenging. Earth is relatively small in size and low in mass, and it orbits far enough from the Sun that liquid surface water is possible. All those factors together make it difficult to find similar exoplanets, so astronomers are happy if they can find worlds similar to Earth in at least one of those three properties.
Hence the excitement over the exoplanet Kepler-78b, a planet slightly larger than Earth but with a similar density—and therefore likely a similar composition. Initial observations measured the exoplanet's orbital rate and its size, but two new papers in Nature provided the first mass estimates. Kepler-78b is about 20 percent larger than Earth and 70 percent more massive, but its density nearly matches that of Earth, indicating that it is likely composed of iron and rock.
THE NAMING OF EXOPLANETS
Exoplanets are named according to the observatory that found them. In this case, the planet is known as Kepler-78b, with its host star known as Kepler-78a and the whole system called Kepler-78. However, the star also carries the names KIC 8435766 and Tycho 3147-188-1 from when it was spotted in other surveys.
This world was discovered by the now-defunct Kepler telescope and was first described in August of this year. Kepler-78b orbits its host star once every 8.5 hours, meaning it's close enough to be tidally locked, presenting the same face to the star just as the Moon does to Earth.
As with other exoplanets discovered using the Kepler observatory, Kepler-78b was found when it transited between Earth and its host star, creating a very slight eclipse that blocked out about 0.02 percent of the star's light with each orbit. Transits are currently the best way to identify smaller exoplanets; the leading alternative locates planets through their gravitational pull on the star, measured as the star's spectrum changes slightly due to the Doppler effect.
Transit and Doppler measurements are complementary. The Doppler method provides a means to measure the exoplanet's mass since it depends on the gravitational effect the planet has on the host star. The transit method allows astronomers to estimate the exoplanet's size: the larger the world, the greater fraction of the star's light it blocks, and vice-versa. Though both methods are biased toward larger and more massive planets, careful observation over many orbits can reveal tiny exoplanets, including one smaller than Mercury.
The best-case scenario is when observers can measure both the mass and size of an exoplanet, since that reveals the density of the world. Astronomers have found that planets 2 to 4 times Earth's radius are fairly common, but their densities cover a wide range of values, running from rocky "super-Earths" to Neptune-like worlds of compressed hydrogen compounds. However, no accurate mass measurements had been made of Earth-size planets to date. (A planet with double Earth's radius may not sound markedly bigger, but it has eight times the volume and four times the surface area.)
To get the mass of Kepler-78b, the two teams of astronomers worked in parallel, using spectroscopic instruments at two observatories: the Keck I telescope in Hawaii and the HARPS-N instrument on La Palma Island near the African coast. (HARPS is the High Accuracy Radial Velocity Planet Searcher, dedicated to Doppler effect exoplanet detection.) The researchers both used the very tiny Doppler shift from the planet's pull on its host star to estimate the mass; that they obtained results close to each other despite working in (hopefully friendly) competition speaks to the trustworthiness of their measurements.
A major difficulty was separating the effects of the planet from other variations in the star's light, such as starspots. (The equivalent of sunspots, these can mimic transits, creating the appearance of exoplanets where none exist. That's why observations over long periods of time are very useful.) To accomplish this task, the researchers estimated the rotation rate of the host star and averaged out the resulting fluctuations before performing Doppler effect measurements. These were very difficult observations, unlikely to be replicated for similarly sized planets orbiting farther out from their host star.
The results between the two teams were fairly close, with error estimates overlapping each other. The Keck group found a planet radius 1.20 times and a mass 1.69 times that of Earth, resulting in a density of 5.3 grams per cubic centimeter. The La Palma researchers measured values 1.16 times Earth's radius and 1.86 Earth's mass, yielding a density of 5.57 g/cm3. Earth's density is about 5.54 g/cm3, with the other rocky worlds (Mercury, Venus, Mars, and the Moon) possessing smaller but similar values, which indicates that Kepler-78b is likely made of similar material: rocks and iron.
The host star Kepler-78a is about 20 percent smaller than the Sun and 600 degrees cooler in temperature. However, with an orbital period of 8.5 hours, the planet skims close enough to the star to have a surface temperature of about 4,800°C, hot enough to melt rock and boil away any atmosphere.
That proximity to its host star means that the planet does not have a long life ahead of it. Tidal distortions of the planet will cause it to lose orbital energy, edging closer to the star until it reaches the Roche limit, at which point the star's gravity will start tearing it apart. This fact alone would suggest that the planet couldn't have formed where it currently resides, but astronomers note another problem: at the time planets formed in this system, the star was larger than it currently is, placing Kepler 78b's orbit inside the star. So something must have happened in the system to shuffle planets around.
This world is not Earth-like in the most meaningful sense: it may have similar chemistry, but it would be more like hell than home. Nevertheless, every discovery like this helps us understand the range of exoplanet types and the formation of star systems.]