The search for planets

How we managed to discover planets orbiting other stars is quite a story.

Actually, how we came to find the planets in our own Solar System is an interesting story too: one that covers our entire history, dating back to when we first started to look hard at the sky and to record what we saw.

At night our ancestors saw the stars wheeling across the sky as the hours passed, saw different constellations at different seasons. They also deduced that what constellations they saw was dictated by whether the Sun was in the sky.

The stars were still there, but invisible in the glare. People did note that under certain circumstances, they could see stars during the day, for example, looking up from the bottom of a dark hole or a hollow tower.

They also noted that the patterns of the stars did not change. However, there were some bright star-like objects that moved against the starry background over days, weeks and months.

The Greeks called them wandering stars, or planets. These wandering stars moved to and fro along a strip of sky called the zodiac. They named them Mercury, Venus, Mars, Jupiter and Saturn. All of these planets are relatively easy to spot.

The scenario of seven objects moving around the sky (including the Sun and Moon) remained for a long time. After all, seven was a number of mystical importance. Then, the telescope was invented.

The early telescopes, like the ones used by Galileo, had relatively poor light-collecting power, and even worse, could only easily observe a small piece of sky.

It was a point-and-observe instrument, not good for searching the sky for new things, although, obviously, Galileo and his contemporaries must have done that. What was needed was a revolution in telescope design.

This happened in 1668, when Isaac Newton made the first reflecting telescope. Instead of using a convex lens to collect the light and form the image, he used a concave mirror.

In addition to not having the false colours and other problems of the lenses available at the time, mirrors could be made big, because they could be supported from behind. Large mirrors mean collecting lots of light and detecting fainter objects.

William Herschel made several large reflecting telescopes and used them to search the sky for comets. In 1781, he found a new planet, which was named Uranus.

Newton's theory of gravity made it possible to precisely analyze planets' orbits, and showed that in addition to planets being pulled at by the Sun, they also tug and tweak at each other, slightly changing their orbits.

Measurements of the motions of the outer planets by Urbain Le Verrier led him to conclude there was another large planet out there. In 1846, Johann Gottfried Galle looked at the predicted position and found the culprit, now known as Neptune, a similar body to Uranus.

In 1906, Percival Lowell, who had dedicated most of his life to the study of Mars, did similar studies of the orbits of the outer planets, including Neptune, and concluded their orbits were being perturbed by an unknown, ninth planet.

Lowell failed to find it. Ironically, he had recorded faint images of this unknown planet in 1915, but hadn't noticed. Finally, in 1930, Clyde Tombaugh did.

However, it turned out that Pluto was too small to explain the orbit perturbations used to find it, so this discovery might have been more due to serendipity and dogged searching than science.

Today, we have more powerful and sophisticated instruments than astronomers in the past could have dreamed of. We can search large areas of sky, look for very faint objects and measure small perturbations in orbits.

We are finding many unknown bodies at the outer edges of the Solar System, but so far, no more planets.

  • In the early hours, Venus lies low in the northeast
  • Mars in the southeast
  • Saturn and Jupiter low in the south.
  • The Moon be new on the 20th.


How can we be alone?

The latest estimate is that there are around six billion Earth-like planets in our galaxy alone.

However, when we really dig into the issue regarding what makes a planet suitable for life as we know it, this large number could be a considerable understatement.

First, we know about places where liquid water and warmth are available for living things, but otherwise they are very un-Earth-like — such as Europa, one of the moons of Jupiter, where tidal forces warm an ocean hidden under a roof of ice.

For the moment, let's just stick to the Earth-like planets. The starting point in identifying an Earth-like planet is that it is the right size, it has an atmosphere, and its surface temperature is high enough to support a water ocean.

There also needs to be a water cycle, where water evaporates from the ocean and returns to it as rain. If there are landmasses, they will be irrigated and material will be eroded from the land and taken into the sea as nutrients for living creatures. However, there is a range of conditions under which this may happen.

First, the planet should be in the Goldilocks Zone, where the planet receives enough warmth and light from its star to ensure a high enough surface temperature and to drive a water cycle.

This is where the situation becomes more complicated. Planets, including ours, exist in a thermal equilibrium. Heat from our star warms our world. As the temperature rises, the Earth radiates increasing amounts of infrared, sending heat off into space.

Eventually, the input and output are equal and the planet's temperature stabilizes. Intriguingly though, if we do this calculation for the Earth, we find our planet should be frozen solid, with a mean temperature more or less equal to the Moon's, around minus 50C.

This obviously isn't the case, and the explanation is the greenhouse effect. Gases such as water vapour, carbon dioxide and methane are greenhouse gases, which means they impeded the ability of a planet to re-radiate heat into space.

The result is that in order to meet a balance of input and output, the planet has to be hotter. Planets with lots of greenhouse gases can be further from their stars and still have comfortable temperatures.

Planets with atmospheres low in greenhouse gases must be closer. The atmospheres of young planets are rich in greenhouse gases.

During the 4.5 billion years since the Earth formed, the Sun has brightened steadily, but on Earth living things removed them and replaced them with oxygen, which is not a greenhouse gas, keeping our environment stable and our planet inhabitable.

In the 1970s, James Lovelock proposed the Gaia Hypothesis (Gaia is the Earth goddess), in which he proposed that once life is established, it has a certain power to keep its environment comfortable.

There are two other factors.

First, there are clouds.

Water evaporated from the oceans by solar heat forms clouds, which can reflect solar energy back into space, providing a stabilizing influence. Of course, more energy in the atmosphere can drive more severe weather.

Second, there is dust.

Every day, warm air heated by contact with warm ground rises, carrying dust with it.  This can act as an insulator, keeping in heat, or as a reflector, sending it back out, depending on the grain size and the amount.

In addition to being the right distance from their stars, we need our planets to have an atmosphere and a signature of water vapour.

If we see oxygen, which needs living things to produce and maintain it, we can be pretty sure there are living things.

Maybe fortunately, the distances between stars ensure it will be a long time before we can interfere with our alien brethren or they with us.

  • Jupiter and Saturn rise in the southeast around midnight
  • Mars follows in the early hours.
  • Venus lies low in the sunrise glow.
  • The Moon will reach Last Quarter on the 12th.

Star will collapse, explode

In science and technology, we are now fairly used to the idea of achieving things today that were unthinkable even a few years ago.

This applies to astronomy, too. One of these is our new ability to make useful images of other stars. Stars other than the sun are no longer inaccessible points of light. We are finding that other stars can be very different from our local, yellow dwarf star. A good example of this is revealed by images taken by the Atacama Large Millimetre Array (ALMA) of the red supergiant Antares, a star in the constellation of Scorpius, which we can see this time of the year low in the southern sky.

ALMA is a radio telescope consisting of an array of 66 dish antennas, which function together as a radio camera. This is an international project in which Canada is a partner. It uses radio wavelengths of a few millimetres (your local FM stations use wavelengths in the region of three metres). At these wavelengths the instrument acts as a very sensitive thermal imager, seeing through the dust clouds that block visible light and infrared. Millimetre wavelengths are strongly absorbed by our atmosphere, especially the water vapour in it, so ALMA is located on one the highest, driest places in the world, the Atacama Plateau in Chile.

Antares is a red supergiant star. It has about 12 times the mass of the sun. The brightness of a star increases enormously as the mass increases. A star with 12 times the mass of the sun will radiate energy at about 5,000 times the rate our sun radiates energy. This means that despite its having more fuel available, it will have a shorter life than the sun. Our star will have a lifetime in the region of 10 billion years. Antares will run out of fuel after a lifetime of about 25 million years. If it has any planets, there is not much chance of life developing on any of them. Antares is now close to the end of its life, and is running out of fuel.

When stars get old, they swell enormously into red giant stars. The sun will do this. With its higher mass, Antares has swollen into a red supergiant star. When we start to run out of something, we generally become more frugal in the way we use it. Paradoxically, stars do the reverse. They burn through their remaining fuel even faster. Antares is now shining about 80,000 times the brightness of the sun. To sustain this, it is totally annihilating 320 billion tonnes of its material every second.   

Antares has expanded to about 700 times the diameter of the sun. Even with 12 times the solar mass, this means the star is about as close to being a very hot vacuum as one can get. Its gravitational hold on its outer layers is weak, and they are flowing off into space as a supersized solar wind. This is where ALMA comes in; it has imaged the outer layers of Antares and revealed how they have become hugely swollen. For example, immediately above the yellow, shining "surface" of our sun, there is a hot layer known as the chromosphere, maybe 2,000 km thick. Antares has pushed its chromosphere out to a thickness of around 500 million kilometres. It is losing material into space at a horrendous rate. This cannot last. When the fuel runs out, the outward pressure will vanish and the star will collapse and then explode. This is likely to happen in the next few thousand years. For a few months, it will outshine everything in the sky other than the sun and moon.

Antares means "Rival of Ares," where Ares is the Greek name for the god of war. His Roman name is Mars.  Both bodies appear as red lights in the sky. However, since stars twinkle and planets don't, it is easy to see which is which. Some time in the next few thousand years, Mars will no longer have any competition. It might worth keeping a weather eye on the southern sky.

  • Before dawn, Jupiter and Saturn are close together in the south.
  • Mars lies low in the southeast.
  • Venus lies low in the sunrise glow.
  • The moon will be full on the 4th. 


How far away is that?

Our telescopes can spot galaxies that formed soon after the birth of the universe. We can measure with great precision whether they are moving towards or away from us. Apart from the nearest galaxies, they are moving away from us, carried by the expansion of the universe. The biggest problem is measuring how far away they are.

To measure the distances of stars in our neighbourhood of our galaxy we use parallax, which is basically a form of the surveyor's technique of triangulation. We take a picture of an area of sky on a certain date. Six months later, when the Earth is on the other side of its orbit and about 300 million kilometres from where the first picture was taken, we take another picture of that same area of sky. Comparing the images will show nearby stars to be in different positions compared with stars in the far background. These position changes tell us how far away those stars are. This procedure works to distances of about 10,000 light years. Since our galaxy, the Milky Way, has a diameter of 100,000 light years and the nearest galaxy like ours is over two million light years away, this technique works only in our cosmic backyard.

In the 19th Century, astronomer Henrietta Leavitt discovered a very useful class of variable star. Because the first one was in the constellation of Cepheus, they became known as Cepheids. These stars cycle in brightness over periods of a few days. This cycle time is easily measured, and Leavitt discovered that the cycle time could be used to calculate the star's actual brightness – its luminosity. So we can measure how bright these stars look, measure the brightness cycle time to get the luminosity, and then calculate how far away that star is. That means if we can spot Cepheids in a distant galaxy, we can work out how far away that galaxy lies. Leavitt gave us a ruler to measure the universe. However, what happens when the galaxy is so far away it is hard to spot individual stars? Then we have to resort to something else – exploding stars (supernovae).

Many stars are double – two stars orbiting one another. In some cases one of them has run out of fuel and has become a white dwarf star. Then, as its more long-lived companion starts to age, and swells into a red giant, the white dwarf starts to pull in material from the other star. When the accumulation reaches a critical amount, there is a nuclear fusion explosion that is visible many millions of light years away. Because these explosions happen when a critical amount of material has accumulated, they are all of more or less the same size. We calibrate nearby events of this type (which are known as Type 1a supernovae) using Cepheids, then use supernovae to measure further out into space, taking us close to beginning of the universe.

Now we have another, even more powerful option – the gamma ray burst. These happen when a really high-energy event happens, like the collapse of a giant star to form a black hole. In a few seconds, more energy is emitted than the sun will produce over its entire lifetime. These bursts of gamma rays have to be observed using satellites because the Earth's atmosphere protects us from them. Once again, we can estimate the energy released, so from measuring what we received here at Earth, we can calculate how far away the event happened. However, the difficulty at the moment is that not all gamma ray bursts are identical. Therefore, this measurement technique still to be calibrated. What we really want is a reliable cosmic ruler that takes us out into space and back in time to about 380,000 years after the Big Bang, where the universe became transparent, and the first stars and galaxies started to form.

  • Before dawn, Jupiter and Saturn are close together in the south.
  • Mars lies low in the southeast.
  • Venus lies low in the sunrise glow.
  • The moon will reach first quarter on the 28th. 

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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