Exoplanet Water Common Yet Rare

Exoplanet water has been found on most planets studied but not in the quantities scientists expected. Find out what this means for the search for life in space.

When my parents bought the forest-covered vacant lot where our family cottage now sits, it was clear what the first order of business was. We needed to drill a well. There were no waterworks within miles of the site.  

We had quickly found that hiking down to the lake and back, hauling five-gallon pails of water, didn’t feel like much of a vacation. Besides, this was long after the days when townships allowed cottagers to use outhouses for toilets.  

We had to have a septic tank, which meant that sooner or later we had to have running water. So, my parents had to bite the bullet and hire a well-driller.  


There is very little soil in that forest bed. They had to drill through about 30 metres of bedrock to release the water hiding in cracks within it. It was a costly and time-consuming job, but we had no choice. That well is now 45 years old and still fulfilling our needs.

Humans can’t get along without water. People often ask themselves what they would eat if there got lost in the wilderness. The answer is that it doesn’t matter very much.  

When it comes to survival, food is a lower priority than we imagine. If we have no air, we will die in five minutes, but that’s seldom an issue. If we have no water, we will die in three days. People have lasted for two months with no food. The first order of business is to find a good source of drinking water.

water one of the main clues in search for life

It’s the same for all living species. No organism on earth can survive without water. Scientists are confident that this is true of exoplanet water as well, although that’s just an educated guess. As a result, water is one of the main clues they look for when targeting exoplanets in the search for life.  

As Dr. Nikku Madhusudhan from the Institute of Astronomy at Cambridge put it, “Given that water is a key ingredient to our notion of habitability on Earth, it is important to know how much water can be found in planetary systems beyond our own,” 

Astronomers tended to assume that there must be planets orbiting stars outside our solar system since Galileo’s time. Despite this, we had to wait until 1992 to confirm this intuitive belief.  Michael Summers and James Trefil describe this history in their book Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life beyond Our Solar System

over 4,000 exoplanets – more than 3,000 star systems

Since researchers detected the very first known exoplanet, they have uncovered over 4,000 new planets in more than 3,000 star systems. Also, astrophysicists can say for sure that almost 700 star systems have multiple worlds just as ours does.

Professor Madhusudhan was a team member on a five-year research program at Cambridge. It’s the most extensive survey of the atmospheric chemical compositions of exoplanets to date.  

They looked at 19 exoplanets and measured their chemistry and temperature. Astrophysical Journal Letters published their results earlier this month.

10 earth masses to 600 earth masses in size

These exoplanets were all much more massive than earth. They ranged from 10 earth masses to 600 earth masses in size.  

They also covered an extensive range of temperatures, from 20˚C up to 2,000˚ C. These were all planets similar to our gas giant neighbours in that their atmospheres are mostly hydrogen.

As the researchers put it, water is “common, yet scarce in exoplanets.” It sounds like a contradiction, but what they mean by this is that it’s quite common to find water vapour in exoplanet atmospheres. At the same time, though, the quantity of water vapour within those atmospheres is strikingly low by our standards.

quantity of water vapour is strikingly low

The team looked at spectroscopic data from the world’s most advanced telescopes, both ground and space-based. These included the Hubble Space Telescope, the Spitzer Space Telescope, the Very Large Telescope in Chile and the Gran Telescopio Canarias in Spain.  

Some or all of these instruments seem to come up in every story we write in our Universe category. We are uncovering an incredible range of knowledge due to their advanced technology and capabilities.

The team measured the level of exoplanet water vapour in the atmospheres of 14 of the 19 planets studied. The atmospheres contained much less oxygen relative to other elements than expected.  

Less oxygen relative to other elements

The reason for this difference seems to have to do with a process called accretion. The planets of our solar system formed through this process when vast quantities of ice, rock and dust clumped together.

It seems that the exoplanet water in the study formed differently. The lack of oxygen in the different atmospheres implies that their accretion processes involved much less ice than in our solar system.

Ever since astronomers first showed that the earth doesn’t revolve around the sun, they have followed something called the Copernican Principle. It started off meaning that we shouldn’t think that we are the centre of the universe.  

copernican principle doesn’t always hold true

More recently, it went even further. Cosmologists started working from the assumption that there is nothing special about our small corner of a garden variety galaxy. However, this doesn’t always hold true.  

What we see in these findings is that we can’t assume that the chemical elements in different atmospheres will be similar to those in our own solar system. This includes volumes of exoplanet water. This challenges the accuracy of multiple current theoretical models.

Ph.D. candidate Luis Welbanks is the lead author of the study. He made an interesting aside. “Measuring the abundances of these chemicals in exoplanetary atmospheres is something extraordinary, considering that we have not been able to do the same for giant planets in our solar system yet, including Jupiter, our nearest gas giant neighbour.”

unable to do this for gas giants in our solar system

The reason for this is that Jupiter is unusually cold. Any water vapour in its atmosphere would be condensed, which has thwarted efforts to measure it.  

This includes NASA’s ongoing Juno mission. Ironically, we’ve measured the volume of exoplanet water in 14 worlds beyond our solar system before we’ve found a way to measure it in a gas giant in our own backyard. 

 The findings of this new study have generated a much higher level of interest in Jupiter’s water vapour. If it’s about what we expected, then scientists’ understanding of accretion is accurate. If not, it will mean that Jupiter’s accretion process wasn’t as ice-laden as planetary astronomers have always assumed.

the highest good is like water

Lao Tzu wrote, “The highest good is like water. For water benefits the ten thousand things without striving.” It turns out that there are 8.7 million species on earth, all of which benefit from water’s lack of striving. Who can say how many extraterrestrial species benefit from exoplanet water?

Dr. Hadhusduhan concluded by saying, “We look forward to increasing the size of our planet sample in future studies. Inevitably, we expect to find outliers to the current trends as well as measurements of other chemicals.”

We always have more to learn if we dare to know.

Learn more:

University of Cambridge
Mass–Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H2O, Na, and K
Tao Te Ching
Astrobiology: 3 Questions We Need to Answer
Exoplanet Giant Orbiting Tiny White Dwarf
Life Began Even Earlier Than Thought
Why Mars? Why Not Life On Venus?



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