Exoplanets between 2 and 4 times the size of Earth may feature water as a large component in their make-up, with many comprising perhaps up to 50% water by weight (by contrast, Earth has just 0.02% water content by weight).
This is the conclusion drawn by an international team of researchers who have being pouring (pun intended) over data from the Kepler Space Telescope and the Gaia mission gathered on the 4,000+ exoplanets discovered thus far, many of which tend to fall into two categories: those with the planetary radius averaging around 1.5 that of the Earth, and those averaging around 2.5 times the radius of the Earth.
It was a huge surprise to realise that there must be so many water-worlds. We have looked at how mass relates to radius, and developed a model which might explain the relationship. The model indicates that those exoplanets which have a radius of around x1.5 Earth radius tend to be rocky planets (of typically x5 the mass of the Earth), while those with a radius of x2.5 Earth radius (with a mass around x10 that of the Earth) are probably water worlds. Our data indicate that about 35% of all known exoplanets which are bigger than Earth should be water-rich.
– Dr. Li Zeng of Harvard University, lead researcher on the study
The teams findings could have major implications for our understanding of the composition of Earth-sized exoplanets. However, if the team’s conclusions are correct, it doesn’t necessarily mean these are especially balmy places. Many orbit so close their parent stars their surface temperatures are liable to be in the 200-500o Celsius range (392-932oF), so the water on them is liable to be very different to how we find it on Earth, existing as saturating vapour in the atmosphere, then a world-girdling warm ocean with ice under increasing pressure below it, wrapped around a sold core.
The beauty of the model is that it explains just how composition relates to the known facts about these planets, and offers insight into how they were formed – most likely in a similar manner to the cores of the giant planets in our own solar system.
With a new generation of Earth-based telescopes capable of peering at distant planets currently gaining remarkable optical updates (such as ESO’s Very Large Telescope) or under construction (the Giant Magellan Telescope or GMT), not to mention the James Webb Space Telescope and the Transiting Exoplanet Survey Satellite (see below for more in this), the hope is that the findings presented by the team will soon be backed-up with hard data as atmospheres around these distant worlds are properly characterised.
TESS Starts Work
TESS, the Transiting Exoplanet Survey Satellite launched on April 16th, 2018, has started its primary mission – taking over from the ailing Kepler mission in locating exoplanets. This initial primary mission will last for 2 years, in which it is anticipated TESS will pay particular attention to the 200,000 brightest stars around us in the hope of detecting planetary bodies in orbiting them. It will do this using the transit method of observation – looking for dips in the brightness of stars which might indicate the passage of an orbiting planet between the star and the telescope.
The first data gathering element of the mission commenced on July 25th, and will continue through most of August before the data is transmitted by to Earth from TESS’s unique orbit, a “2:1 lunar resonant orbit“, which allows the craft to remain balanced within the gravitational effects of the Moon and Earth, providing a stable orbital regime which should last for decades.
As a part of the mission, the TESS science team aims to measure the masses of at least 50 small planets whose radii are less than four times that of Earth, offering the opportunity to characterise their likely structure and composition. Many of TESS’s planets should be close enough to our own that, once they are identified by TESS, scientists can zoom in on them using other telescopes, to detect atmospheres, characterise atmospheric conditions, and even look for signs of habitability.
In this latter regard, TESS will pave the way for detailed studies of candidate exoplanets by the James Webb Space Telescope (JWST), now scheduled for launch in 2021. While TESS cannot look for atmospheric or other signs of life on the distant worlds it locates, JWST will be able to do just that, which could see the 2020s a decade of remarkable extra-solar planetary discoveries.