At the start of 1995, we knew of only 9 planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Although we have since lost Pluto, we have now confirmed over 3,700 exoplanets — planets orbiting a star other than our sun. These exoplanets have been discovered by various methods, but the vast majority have been detected via indirect methods — measuring the influence of an exoplanet on its host star. We have also managed to directly image a number of exoplanets. This is the most difficult technique since most planets are lost in the bright glare of their host star. In recent years, we have discovered that in addition to the exoplanet companions — exoplanets orbiting a host star, there have been a number of discoveries of so-called rogue, or free-floating planets. These are planetary-mass objects (less than about 13 times the mass of Jupiter) with no host star, wandering the Milky Way alone!
There are currently two theories about the formation of these isolated planets. The first theory suggests that they form similar to a star like our Sun — through the collapse of a massive interstellar cloud composed of molecular gas and dust. Once enough material is compressed at the centre of the cloud, nuclear fusion is ignited in the core, and a star is born. Once nuclear fusion is established, a star will continue to shine for about 10 billion years. However, in the case of our free-floating planets, we think that the core did not accrete enough material to trigger nuclear fusion. These objects can be thought of as ‘failed stars’, and spend their entire lifetimes cooling down. The other theory proposes that the free-floating planets were ejected from a planetary system. This can happen due to gravitational interactions with other planets within the system or a close encounter with another star. These interactions could fling a planet out of its orbit and leave it free to travel through interstellar space. Most likely, the free-floating planets that we have discovered to date formed through both of the theories discussed here, but we have not yet found a way to distinguish them from each other.
Free-floating planets pose a huge advantage to astronomers studying exoplanets. The population of free-floating planets share a remarkable resemblance with the small population of directly-imaged planets that we have discovered. The free-floating and companion exoplanets share similar masses, temperatures, ages and sizes, but while the companion exoplanets are extremely hard to image, the isolated planets are much easier since they do not have a bright host star nearby. New instruments and technologies are currently being developed so that we may study companion exoplanets in detail in the future. In the meantime, the free-floating objects can be studied in exquisite detail and act as useful analogues for the directly-imaged companions, providing clues on what we might expect.
Brightness Modulations Signal Atmospheric Features
While it can take several hours to obtain an image of an exoplanet orbiting its host star, a medium-sized telescope can capture images of a free-floating planet on ~5 minute timescales. We cannot resolve the surface of a planet since it is too far away, but we can make use of the fact that they rotate to try to identify the presence of weather patterns in the planet’s atmosphere. This is done through a technique called ‘photometric variability monitoring’, which basically means measuring the brightness of an object over time. By monitoring the brightness over many hours we can approximate what the upper atmosphere of such an object looks like. The video below shows an artist’s concept of a brown dwarf with atmospheric bands of clouds, thought to resemble the clouds seen on Neptune and the other outer planets in the solar system. The dots on the bottom show the measured brightness of the planet over time, called the lightcurve of the planet.
PSO-318.5-22: A Cloudy Free-floating Planet
In 2015 I used the New Technology Telescope in La Silla, Chile to observe the free-floating planet PSO J318.5-22. PSO 318.5-22 is a free-floating planet situated 80 light-years from earth, with a temperature of 800°C and a mass 7 times that of Jupiter. This object is unusually red compared to other objects with similar temperatures, and this is thought to be due to the presence of very thick clouds in its atmosphere. Using the images, we could measure the brightness of this object in each frame, and found that the brightness of this isolated planet changed by up to 10% over the course of 5 hours. Follow-up observations showed that the lightcurve is periodic, repeating itself every ~8.6 hours, indicating that this was the rotational period of the planet. Every 8.6 hours an atmospheric feature, most likely silicate clouds and iron droplets, would rotate in and out of view. This was the first detection of weather on an planetary-mass object, and hinted that these atmospheric features may be common on extrasolar planets.
We then went on to observe PSO J318.5-22 simultaneously using the Hubble Space Telescope and the Spitzer Space Telescope, which allowed us to track the brightness of our target in a variety of different wavelengths with unprecedented accuracy. The new lightcurves revealed that although all lightcurves showed brightness modulations in agreement with a 8.6 hour rotational period, the lightcurves obtained from the Hubble and Spitzer telescopes appeared ‘out of phase’. This means that when the planet appeared at its brightest in the Hubble images, it appeared very faint with the Spitzer Space Telescope, and vice versa. The Hubble and Spitzer Telescopes differ in the wavelengths they use — Hubble observations are in the near-infrared while Spitzer probes longer wavelengths in the mid-infrared. Different wavelengths are sensitive to different heights in the atmosphere of the planet — the Hubble telescope sees deep into the planet’s atmosphere while the Spitzer wavelengths only see the highest altitudes. The observed shifts between lightcurves suggest that we are observing different layers of clouds located at different vertical positions in the atmosphere. These types of observations have thus allowed us to explore both the horizontal and vertical cloud structure of PSO 318.5-22, a rogue planet lying 80 light-years away.
Future Exoplanet Companion Studies with JWST
Now that we have developed the technique of photometric variability monitoring, we hope to extend these studies to the directly-imaged exoplanet companions once the James Webb Space Telescope (JWST) launches. Due to be launched in 2020, JWST will revolutionise all fields of astronomy by providing unparalleled sensitivity to astrophysical signals at a wide range of wavelengths. JWST will allow us to extend the variability monitoring discussed above to exoplanet companions, such as the HR8799bcde planets shown below. This system of four planets, called HR8799bcde is so far the only multi-planet system that has been imaged. By re-observing the HR8799bcde system over a number of years, astronomers could track their movement around their host star. The four planets shown here share very similar properties to free-floating planets such as PSO J318.5-22, and so we expect that they will show similar brightness changes over time. Current telescopes cannot obtain images of these planets at the sensitivity and cadence needed to measure photometric variability, but JWST will allow us to carry out these measurements for the first time.