Many of the 4,000+ planets so far discovered in distant star systems in the Milky Way are “water worlds” according to new research. The study into the distribution of planet sizes in other systems concludes that many planets found that are bigger than Earth, but smaller than Neptune, may be “water worlds”. It comes days after scientists also found cold gas giant planets.
“Statistically speaking, these water worlds may be more abundant than Earth-like rocky planets,” says Li Zeng, Simons Postdoc Fellow at Harvard University, whose Growth model interpretation of planet size distribution paper was published today. Its simulations suggest that sub-Neptune exoplanets, which have radii two to four times that of Earth, likely contain at least 25% ices or fluids. It had been thought they were gas dwarfs with a rocky core surrounded by a gaseous envelope. “Perhaps every typical sun-like star has one or more of these water-worlds … perhaps our solar system is less typical,” says Zeng.
How many water worlds are there?
At least one thousand, and that’s just in 4,000+ exoplanets found by the Kepler Space Telescope, which gathered data from a few areas of the Milky Way between 2009 and 2018. “Due to the geometry of the transiting method, we can only see planetary systems with their orbital planes aligned along our line-of-sight,” says Zeng, who adds that there will be many more hidden planetary systems out there. “The current estimate is that every star in our galaxy has its own planetary system … planet formation is a universal physical process accompanying star formation.”
Zeng stresses that what Kepler has found, and what the TESS all-sky survey is already finding, is just the tip of the iceberg.
Why is this research important?
Astronomers take the solar system as a template to work with when studying other star systems, but increasingly it’s becoming apparent that the solar system may be an odd one out. Planet formation theories based on the solar system are being challenged more and more. Zeng’s results backs-up a direct observation in February 2019 of ice in the planet-forming disk around a star called V883 Ori 1300 light-years away from the Earth.
How were these water worlds discovered?
Zeng developed a method of simulations of the mass and radius distributions of exoplanets that considered:
- How water-ice behaves under high pressures and temperatures inside a planet
- Analysis of observational data on the mass and radius of planets
- The chemistry of how planets form
- How the planets’ atmospheric gases may have escaped into space
The research also made use of the very latest data from Gaia via Travis A. Berger in the University of Hawaii. Gaia measured the motions of the stars on the celestial sphere with high precision, confirming absolute measurements of the distances to stars close to us in the Milky Way. “Many uncertainties in the measurements of planet radii previously resulted from our unknown of precise stellar radii, which in turn is a result of inaccurate distance estimates of those host stars,” says Zeng. “Gaia solved it.”
Why are there no ‘water worlds’ in our solar system?
You can have Jupiter, or you can have water worlds. Probably not both. “The formation of gas giants and the formation of those close-in super-Earths and sub-Neptunes are somewhat mutually exclusive,” says Zeng. “Our solar system had formed the gas giant Jupiter early on, which probably had prevented or interfered with the formation and growth of super-Earths and sub-Neptunes.” However, in other star systems where no gas giants formed early on, water worlds are possible. “Probably the physical and chemical conditions in the proto-planetary disk are favorable for the formation of those super-Earths and sub-Neptunes,” says Zeng, who adds that close-in rocky super-Earths and water-rich sub-Neptunes may be common in the Milky Way galaxy.
Why is it so difficult to find Earth-sized water worlds?
Size, distance, and limited instrumentation. After all, planets are much smaller and fainter compared to their host stars, so much so that they cannot be imaged directly … yet. “The host stars themselves are fluctuating all the time, produce variability in their light, which is much stronger than the planet signal itself,” says Zeng. It’s, therefore, a noisy signal. “The mainstream methods, both the radial velocity (RV) method and the transiting method, are indirect methods, which rely the detection of planets upon their (small) effects/influence on the stellar light.” For now, planets are inferred by data, rather than directly discovered.
What will TESS discover?
TESS is big news in for exoplanet-hunters. “In the near future, the TESS mission will find many super-Earth sized planets, and probably a handful of Earth-sized planets as well,” says Zeng. Observing 85% of the sky containing more than 200,000 nearby stars, researchers expect to identify around 20,000 exoplanets from TESS data. “Ground-based follow-ups using mainly the radial velocity (RV) method, will be able to measure some of their masses … the RV method can be improved with a higher-precision instrument and a better understanding of stellar noise.”
Zeng’s research was supported by the Harvard Origins of Life Initiative, directed by Prof. Dimitar D. Sasselov and the Simons Foundation, funded by the generosity of Jim Simons.
