Water, water everywhere

University College London: 11-Jul-2007 13:00 Eastern US Time

Scientists have found water on the planet of another star. This is the first time this key substance for living things has been found on an extrasolar planet.

More than 200 extrasolar planets have been discovered so far. These are planets in orbit around a star, in the same way as our Earth is in orbit around the sun.

Extrasolar planets don't usually have interesting names like Venus or Pluto. This one is called HD 189733b. It orbits a star in the constellation of Vulpecula the Fox. It is 64 light years away from us.

HD 189733b is a "transiting planet". This means it passes in front of its star. As transiting planets do this they absorb light from their star.

Different substances absorb different colours of light. So scientists can study the light that comes through their telescopes and discover what the planet's atmosphere is made of.

The team of scientists who discovered water on HD 189733b was led by Dr Giovanna Tinetti. She is a scientist at the European Space Agency and University College, London. The findings will be published in this week's Nature (July 12).

This is the first time that astronomers have been sure there is water on an extrasolar planet. Unfortunately the water is not lying around in puddles in great lakes and oceans full of alien fish and seabirds. We know this because HD 189733b is very hot indeed. So water there is in the form of water vapour in the atmosphere of the planet.

HD 189733b is far from being habitable, says Dr Tinetti, who recently took up an Aurora Fellowship at UCL. In fact it is quite hostile to life. But the new discovery shows that water might be more common out there than scientists thought.

It also shows something very important for future work, she says. The same method Dr Tinetti and her colleagues used to detect water on HD 189733b "can be used in the future to study more 'life-friendly' environments."

NASA's Spitzer Earth-orbiting telescope was used to make the discovery. The scientists made measurements at a number of key wavelengths. These were in the infrared part of the spectrum. It is light at these wavelengths that water vapour absorbs.

The detection relied on Dr Tinetti's painstaking analysis. It also relied on calculation of very accurate water absorption parameters. Dr Bob Barber and Professor Jonathan Tennyson did this. They are in UCL's Department of Physics & Astronomy.

Dr Barber said: "The absorption parameters were calculated from our Barber-Tennyson list of water vapour spectral lines. This includes over 500 million individual absorption features."

Each of these is like a fingerprint, he added. They provide "vital clues to the amount of water present and the temperature of the atmosphere."

Parts of the atmosphere of HD 189733b are very hot - around 2000 degrees Celsius, said Professor Tennyson. He is head of UCL's Physics & Astronomy Department. "You need the millions of lines we calculated to simulate this."

HD 189733 is a star much like our own Sun, although a little cooler. But its planet is very unlike Earth. It is a gas giant like Saturn and Jupiter, the largest planet in our solar system. It is actually 15% bigger than Jupiter.

The main difference between our gas giants and HD 189733b is distance from their parent sun. Jupiter is over five times as far away from the Sun as Earth is. But HD 189733b is more than 30 times closer to its star than Earth is to the Sun. This is why HD 189733b is so much hotter than Jupiter.

The "holy grail" for planet-hunters is to find a planet like Earth that has water in its atmosphere, said Dr Tinetti. "When it happens, that discovery will provide real evidence that planets outside our Solar System might harbour life.

"Finding the existence of water on an extrasolar gas giant is a vital milestone along that road of discovery."

More help with words

electromagnetic waves

feature

identifies

microwaves

model

planet

universe

What's it all about?

1. What have the scientists discovered?
2. What is an extrasolar planet?
3. What is the name of the extrasolar planet that has water on it?
4. How far away from us is this planet?
5. What does a transiting planet do to light from its star?
6. So scientists can study this light to find out which colours are missing. This tells them what substances have -------- the light.
7. Is the water on this planet lying around as a liquid?
8. What form is it in?
9. Could people live on HD 189733b?
10. What phrase from the article gave you the answer to the last question?
11. This discovery does not show there could be life on HD 189733b. But it does show two things. State one of them.
12. Which part of the electromagnetic spectrum were the scientists particularly interested in?
13. In your own words and one sentence explain why.
14. There were two main parts to the work that led to this discovery. One was to collect light from the star using the ------- telescope.
15. Then the scientists had to study this light to see what "colours" or wavelengths it contained - more importantly which ones were missing because they had been absorbed. Was this second part easy or difficult?
16. Which word in the story helped you to answer the last question?
17. Dr Barber mentions two other things about the planet - apart from water vapour being there - that can be learned from the light. What are they?
18. In what way is HD 189733b like Jupiter?
19. In what way is HD 189733b unlike Jupiter?
20. HD 189733 and HD 189733b are names for two different things. Look at where these names are used in the story and explain what each of them means.
21. In one sentence explain why astronomers are so interested in finding water on other planets.
22. If you were these scientists what work would you like to do next?
23. What question would that research be trying to answer?

More activities for this story

Water everywhere UK US

Topic for discussion, research or pupil presentation

A) I chatted to Dr Tinetti about her research earlier this summer, when she first came to London looking for somewhere to live while working at University College. After explaining her approach to searching for life on exoplanets - in which she develops computer models of planetary atmospheres and compares them with observations - she tried to answer a harder question: "Do you think there is life out there on other planets?"

Working in groups, students should listen to the short audio extract of this part of our talk. During the rest of the interview, which lasted over an hour, Dr Tinetti talked confidently about the scientific methods she and her colleagues are using - analysis, modelling, making observations. But in this extract she suddenly starts using a particular phrase that makes her sound very hesitant.

Working in groups students should try to find that phrase, and count the number of times Dr Tinetti uses it or some variant of it. They should then try to decide why the scientist seems so unwilling to be definite when tackling this one question.

The clue, which the teacher might point groups to as necessary, comes right at the end of the recording.

B) More activities:

Teachers' Domain has an appealing set of interactive resources on the search for extraterrestrial life, the Spitzer Space Telescope and infrared astronomy, astronomy at different wavelengths and the importance of water to life. These are pulled together in a comprehensive lesson plan on what makes a habitable planet. A teacher or school needs to register to access this, but this is a simple process from here.

C) Centauri Dreams is probably the best place on the Web for news and informed opinion
on exploring the stars and the prospects of finding life on other planets.

Tips for science class discussions and groupwork

No 55

Science inquiry is typically a collaborative activity and as such involves teams of students in discussing, planning, and conducting investigations together and in sharing responsibilities for talking, reading, writing, and other kinds of presentations. Communication, therefore, plays a major role in science inquiry, and language is one of its central elements. In science inquiry, however, communication involves simultaneous use of other forms such as pictorial and numerical representations. Teams of science inquirers talk about and write their questions, their tentative explanations, their plans, their data, their conclusions, and their reasons and judgements about relationships between evidence and explanations, and about how they make public presentations and scientific arguments in behalf of their work. It is in the context of this kind of scientific activity that students' literacy of the spoken and written word develops along with literacy of the phenomenon. It is also in relation to direct experiences in scientific investigations that words acquire nuances or "negotiated meanings"; inevitably, those meanings differ from meanings conveyed by the same words in ordinary speech.

From Douglas, R. (ed.) et al. (2006) Linking Science & Literacy in the K-9 Classroom. Arlington: NSTA Press.