Habitable planet found

First habitable Earth-like planet
Astronomers have discovered the most Earth-like planet yet outside our solar system. It has a radius just half again as big as Earth’s. Conditions there are suitable for liquid water. So there could be life on this planet.

The star is called Gliese 581. It is among the closest of stars to us, just 20.5 light years away in the constellation Libra. It is a red dwarf.

These are small and faint, but they are the most common stars in our galaxy. Among the hundred closest stars to us, eighty are red dwarfs.

A team of Swiss, French and Portuguese scientists discovered the new planet using the European Southern Observatory 3.6-metre telescope. They also have strong evidence for the presence of a third planet with a mass about eight times the mass of the Earth. Gliese 58 was already known to have a planet, a gas giant about the same mass as Neptune.

This new exoplanet, which is what astronomers call planets in orbit around stars other than the sun, is the smallest yet found. It completes a full orbit in 13 days. It is 14 times as close to its star as the Earth is to the sun.

But Gliese 581 is much smaller and colder than the Sun. This makes it far less bright. So although the planet is so close it does lie in the habitable zone. This is the region around a star in which water on a planet would be liquid, rather than steam or ice.

"Liquid water is critical to life as we know it," says Xavier Delfosse. He is a member of the team from Grenoble University (France).

Because of its temperature and nearness to Earth, this planet will be an important target for future space missions looking for extraterrestrial life, he added. "On the treasure map of the Universe, one would be tempted to mark this planet with an X.

Red dwarfs are ideal targets for the search for planets like Earth, says Xavier Bonfils, of Lisbon University. "Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun."

Planets close to their suns are much more easily detected with the radial velocities method, the most successful method for detecting exoplanets. This looks for wobbles in the motion of the star caused by the planet, by studying small changes in the star’s light.

"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explains Stéphane Udry. He is from Geneva Observatory (Switzerland) and is lead author of the paper that reports the result.

Its radius should be only 1.5 times the Earth’s radius, he added. "And models predict that the planet should be either rocky, like our Earth, or covered with oceans."

The same team of astronomers found a planet around Gliese 581 two years ago. It is 15 times as massive as the Earth. It is similar to Neptune, and has a year that is just 5.4 days long. At the time, the astronomers had hints of another planet. So they obtained a new set of measurements.

With these they found the new super-Earth. They also found clear indications of another planet, around 8 times the mass of Earth. This completes its orbit in 84 days. This means that the planetary system surrounding Gliese 581 has three planets of 15 Earth masses or less. The discovery was made using HARPS (High Accuracy Radial Velocities for Planetary Searcher). This is perhaps the most precise spectrograph in the world. It is located on the ESO 3.6-m telescope at La Silla, Chile.

HARPS can measure velocities with a precision better than one metre per second. It is one of the most successful instruments for detecting exoplanets, and holds several recent records. These include the discovery of a ‘trio of Neptunes’.

The new planet was detected through small changes it caused in the speed of motion of its star. These were only about 2 to 3 metres per second. This is the speed of a person walking fast. Such tiny signals could not have been detected with most spectrographs.

"HARPS is a unique planet-hunting machine," says Michel Mayor, from Geneva Observatory. He is the HARPS principal investigator. Of the thirteen known planets with a mass less than 20 Earth masses, eleven have been discovered with HARPS, he added.

"We are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach."

This research is reported in a paper submitted as a Letter to the Editor of Astronomy and Astrophysics.

What’s it all about?

Plenty of planets have been found outside our solar system. This one is special for several reasons. State one of them.

The new planet is in orbit around a particular type of star. Which type of star?

These stars are small, faint, but very ______.

What type of instrument did the astronomers use to discover the new planet?

How many planets, that we now know of, are there around Gliese 58?

A year is the length of time a planet takes for one complete orbit of its sun. How long is a year on this new planet?

The new planet is much closer to its sun than Earth is. So why is it not baking hot?

If a planet is too close to its sun any water on it is in the form of steam. If it is too far away any water is in the form of ice. What do we call the region in between?

Why is it given this name?

Delfosse says we should mark a treasure map of the Universe with an X to show this planet. In one sentence what does he mean?

What is the name of the most successful method for finding exoplanets?

This method can find planets much more easily if they are _____ to their sun.

The habitable ____ is closer to a ___ _____ type of star than to stars that are bigger and brighter.

This is why it is easier to find planets in the _________ zone of a red dwarf star.

And so this means that red dwarf stars are a good place to look to find extraterrestrial ____.

What is the mean temperature of the new planet?

Almost all the exoplanets found so far are mostly gas and have no solid surface. Why is this new planet a much better place to look for life?

The HARPS instrument is part of the ESO 3.6-m telescope. What was HARPS particularly designed to do?

HARPS is used to study the light from a star. Small changes in the colour of this light are caused by small changes in the motion of the star. How big were the changes in the motion of this star that showed it had a planet?

Most exoplanets discovered so far are enormous. This is because large planets are easier to detect than smaller ones. But they are not as interesting. Why not?

If you were these scientists what questions would this new discovery raise in your mind?

Can you think how you might try to answer those questions?

What kind of story is this?
Learning to do science is about learning to think. Experiments, direct teaching, group activities and discussions all have a part to play. So do science news stories.

Like other non-fiction texts, science stories contain different kinds of statements. To get at the science behind the words - and to make reading them an active experience - students should pull a text apart and explore the kinds of statement it contains.

We've met some of these in the later questions of the previous activity. Science news stories usually include the aims of the research or reasons for doing it. They often contain a hypothesis. Sometimes evidence for a hypothesis is given, or a hypothesis is used to make a prediction. Towards the end of a story the direction of future research the scientists are planning is often discussed, as well as outstanding questions the research will be designed to answer.

All these types of statement occur in some science stories. Virtually all science stories, however, will contain statements of the following four types:

• new findings or developments;
• the technology and methods the scientists used;
• previous or accepted knowledge, which may or may not be supported by the new findings;
• issues, implications and applications of the research.

So the next activity is designed to engage students with the latest science news by exploring the meaning and structure of a story as revealed by the content and balance of these four statement types:

Pulling it apart

In groups students should read through the story looking for new findings or developments. Once they have reached agreement, or at least consensus, and have underlined all the statements about what the scientists have just discovered or achieved, they can compare their thoughts with ours

In groups they should go through the story again looking for the technology and methods the scientists used in their research. Once they have reached agreement or consensus, and have underlined the statements that talk about the methods and equipment the scientists used, they can compare their thoughts with ours.

They should repeat the activity for existing knowledge and compare their thoughts with ours.

Any areas of disagreement in these activities - whether among the students, between teacher and students, or between the class and our own ideas - should be regarded as opportunities for discussion rather than errors to be corrected.

Having fully engaged with the latest science news through the above activities, students will be far better able to talk and think about the science and its implications than someone who has simply read about it in a newspaper or watched a brief item on television.

Now it's time for them to get to grips with the issues raised by the research.

Young people have opinions. But school science traditionally allowed little scope for forming and expressing these - which is why it turned many of them off the subject for life.

Putting it together again

In groups, students should read through the latest story looking for issues, implications and applications. Once they have reached agreement, or at least consensus, and have underlined all the relevant statements in the story, they can compare their thoughts with ours.

Having done all this the students are well armed to explore the issues raised by the story. A suggested discussion topic specific to this new story is provided below.

Topic for group discussions, web research or pupil presentations on the new exoplanet discovery

1. Discuss what effect the discovery of life on other planets would have on how people think about science. Would the subject become more popular? Would religions with their teaching that Earth humans are special fade away or be drastically modified? Would the answers to these two questions be different if the extraterrestrial life we discovered were intelligent beings like us or "just" micro-organisms?

2. There are now scientists called astrobiologists who aim to discover if there is life on other planets, by analysing the light that comes from them. Amazingly they are preparing to do this for exoplanets such as this latest discovery. Separating the light from these planets from the light from their stars has been compared to spotting a candle placed beside a lighthouse that is 1000 kilometres away. Students should read an interview with astrobiologist Giovanna Tinetti then in groups prepare a short presentation on what an astrobiologist actually does.

Tips for science class discussions and groupwork

No 48
...there is a distinct trend to make better use of practical activity through class and group discussions both on the evidence for particular points of view and on the social issues concerned with the application of science. This technique of encouraging reasoned discussion amongst pupils is borrowed from successful practice in social studies and recognises the constructivist principle that learning is not just a matter of absorbing information, nor even of discovering information, but involves a paradigm shift in belief by learners which is most likely to be brought about by open discussion.

Adey, P. and Shayer, M. (1994) Really Raising Standards. London and New York: Routledge.