Two planets originally discovered by the Kepler mission may not be what they thought, reports Ars Technica. Based on the initial data, there were assumptions that they are rocky bodies slightly larger than the Earth. But further observations indicate that they have a much lower density, which is characteristic of planets with water or a liquid of similar density.
There are similar bodies in our solar system – in particular, Jupiter’s moon Europa, which has a rocky core surrounded by a watery shell covered with ice. But these new planets are much closer to their star, which means their surface is likely a blurred boundary between a vast ocean and a vapor-filled atmosphere.
There are two main methods of finding exoplanets. One of them is to observe dips in the light from the star caused by planets with an orbit that passes between the star and the Earth. The second is to monitor whether the star’s light periodically shifts to red or blue wavelengths, caused by the star’s motion due to the gravitational pull of orbiting planets.
Any of these methods can tell us whether or not a planet exists in orbit around a star. But having both gives us a lot of information about the planet. The amount of light blocked by a planet can give us an estimate of its size. The amount of redshift and blueshift of a star’s light can indicate a planet’s mass. With the help of both of these indicators, we can find out its density. And density limits what materials it can be made of—low density means it’s rich in gas, high density means it’s rocky with a metal-rich core.
This is exactly what scientists were able to do with the example of the Kepler-138 system. Data from both of these methods suggested that the system contains three planets. Kepler-138b is a small, Mars-sized, rocky body. Kepler-138c and Kepler-138d fell into the category of super-Earths: rocky planets that were slightly larger than Earth and significantly more massive. All of them orbit quite close to the star Kepler-138a, a red dwarf, and the most distant one (Kepler-138d) is at a distance of 0.15 AU (the typical distance between the Earth and the Sun).
By and large, there was nothing unusual about this system that would require further study. But researchers thought it was a good candidate for determining the presence of an atmosphere. While a planet blocks all light as it passes in front of its star, a small amount of light passes through its atmosphere on its way to Earth. And the molecules in this atmosphere absorb some specific wavelengths, which allows scientists to recognize their presence.
To conduct this study, the research team obtained data from the Hubble and Spitzer space telescopes as Kepler-138d passed by the star. And that’s when strange things started.
The three planets clustered in a small region near the red dwarf are close enough to influence each other’s orbits. This creates so-called “transit timing variations”, which means that the planet does not appear in front of its star at exactly the time when its orbit should have brought it to that point. For example, one of the planets may find itself in such a position that its gravitational attraction slows down the other, as a result of which its transit will begin a little later than the calculations would suggest.
It can also impose limits on planet mass estimates, so it is very important to have accurate measurements of transit time variations. And, because the Hubble and Spitzer observations were made some time after the Kepler data, this meant that scientists could calculate variations over a seven-year period.
However, as it turned out, they could not. Estimating the mass from the Kepler measurements and then trying to use it to predict transits in later measurements didn’t work. In fact, everything was mixed up.
When the three-planet model didn’t work, the researchers had a fallback: try the four-planet model instead. And this made it possible to make sense of the data. It also made it possible to estimate the location and mass of a fourth planet: about half the size of Earth, orbiting at a distance of about 0.2 AU from the star. The Kepler-138e planet does not appear to transit in front of the star, so its presence has not yet been confirmed.
However, assuming it exists, the presence of Kepler-138e has some implications. After all, the planet also exerts a gravitational pull on the star, which contributes to all the red and blue shifts in the star’s light that were used to determine the mass of other planets. Thus, all mass estimates based on previous data had to be completely revised in light of the presence of another planet. And when that was done, things continued to get weird.
The two larger planets, Kepler-138c and Kepler-138d, were originally thought to be quite different: both rocky, but with metallic cores that differed greatly in size m. However, with revised measurements, they turned out to be essentially twins. And they were much less dense than according to previous estimates.
One hypothesis for this is that they have a large, hydrogen-rich atmosphere. But the planets are so close to their star that this is not a viable option. The radiation from the star is intense enough to destroy the atmosphere in 50 million years, and the system is estimated to be over a billion years old.
An alternative is a planet rich in so-called volatiles, such as water or ammonia, which can be found as gases, ices and liquids under the conditions found in various parts of the solar system. While a number of potential chemicals could explain the planets’ density, researchers are thinking about water because there are several water-rich worlds in our solar system, including Jupiter’s moon Europa.
Comparing the densities of the two planets yields a model in which just over 10% of the planet’s mass consists of water. This, however, means that about half of the planet’s volume is water. Although part of it may be included in the rocky core, this most likely means the presence of an ocean that is a kilometer deep. And, unlike icy moons, the planet is close enough that most of the water would be liquid and the atmosphere would be filled with water vapor.
The water-filled moons of the outer solar system are easily explained because they formed in a region where water could exist as ice, and thus could condense into smaller bodies that coalesced to form moons. But these planets orbit in a region where water is either liquid or, more likely, remains gaseous.
Obviously, given that researchers have not confirmed the existence of a fourth planet, there is much more to be verified before it can be said with certainty that water worlds have been found. But even in its current preliminary state, the results show that there is still plenty of potential for new discoveries in places where the data would seem to have pointed to a rather mundane collection of planets. Given that Kepler has discovered thousands of similar exosolar systems, there is enormous potential to sift through the data and find new surprises.