PLANETS, PLANETS EVERYWHERE…

It is a remarkable thing to think about. Just twenty four years ago we knew of only nine planets (Pluto had yet to be demoted) and they were all in our solar system. The first planets to be detected were found in 1992 orbiting a very unusual object, a pulsar called PSR1257+12. Three planets were discovered orbiting this rapidly rotating neutron star. (The name references its position in the sky.)

A neutron star is the remains of a large star that has become a supernova, but was too small to form a black hole. Instead it leaves behind a dense core. Pulsars are a form of neutron star which has a very powerful magnetic field which can be detected from earth if the beam of radiation passes in our line of sight. They are nick-named ‘cosmic lighthouses’ as the beam of radiation spins past very regularly, as does the light from a lighthouse. When they were first discovered in 1967 by British astronomer Jocelyn Bell the regularity lead to them being thought of as artificial as it was too regular for a natural event.

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The paper trace of the discovery of the first pulsar, spinning at a rate of once every 1 1/3 seconds. (www.jb.man.ac.uk)

Then, on the 6th. October 1995 two Astronomers in Geneva announced the discovery of a planet orbiting 51 Pegasi, a main sequence star similar to our own.

From then on the discoveries of new worlds gathered pace, initially large planets, some bigger than Jupiter were the ones being found. The techniques in the ‘early days’ of discovery meant that they were found only because of their size and proximity to their host stars. This led to their being called ‘Hot Jupiters.’

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If you want to find the star 51 Peg it lies very close to the Square of Pegasus and is visible easily in the northern hemisphere.

Very quickly, though, smaller and smaller planets were discovered. Today (28th. August 2016) 3,518 exoplanets are known (for details visit exoplanet catalogue at www.exoplanet.eu ). Sizes vary for these planets as can be seen by this NASA graphic:

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Of course as soon as planets were found the hunt then began for planets that might support life. As far as we understand, life needs amongst other things a planet between one and 10 times the mass of the Earth. It needs to be big enough to hold onto an atmosphere but not so big that here is too much hydrogen and there needs to be water. So we need to find planets where water was likely; that means they would have to lie in a region which was not too close to its star (where it would be too hot for water and possibly life) and not too far (where it would be too cold for liquid water) but in a region that was just right. This region is the Habitable or Goldilocks zone.

Now, depending on the size and age of a star the Goldilocks zone will vary; around a cool, red dwarf a planet would need to be much closer than around a star like the Sun. A number of good candidates have been found; (although there is some doubt about Gliese 581g as there have been no successful follow-up observations of planets. This is a criterion that needs t be met to ensure that the discovery is not a ‘false positive.’)

How do Astronomers look for an exoplanet? Well there are a number of techniques; they can be detected directly and be imaged (this is the most difficult method) or they can be detected indirectly, discovering the existence by the effect on the host star.

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The direct detection method of imaging an exoplanet is difficult because a planet (even a super sized ‘Jupiter’) is very dim when compared to a star. The star literally outshines its planets. To try and see planets this way it is necessary to block out the star’s light. To do this an instrument called a Chronograph is used. It covers the disk of the star but leaves the outer regions of its atmosphere visible allowing any planets to shine through. Think of a total solar eclipse and how, when the Moon covers the Sun completely we can see the outer layers of our star’s atmosphere, this is a natural chronograph.

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Total Solar eclipse showing the Corona, the Sun’s outermost layer.

Observing a planet this way is vital though if we are to try and examine its atmosphere to determine how much water it has and if there are any chemical signs of life. (In 2010 the European Southern Observatory’s (ESO) Very Large Telescope (VLT) captured the first direct spectrum of an exoplanet!)

There are a number of indirect ways of detecting exo-planets including watching for planets transiting the host star. A transit occurs when an object passes in front of another object; from Earth we see this when Venus or Mercury pass in front of the Sun. When an exo-planet transits its host it dims the star ever so slightly. By measuring how much the star is dimmed (using an instrument known as a photometer) and how long the transit lasts Astronomers can work out the size of the planet, its orbit and the composition of the planet’s atmosphere.

Spacecraft like the Kepler telescope and COROT use this method and have had much success in finding new bodies. Future missions like ESA’s Darwin and NASA’s Terrestrial Planet Finder will search for oxygen, carbon dioxide and chlorophyll. These missions are planned for launch in the next decade.

The big news though is that an Earth like planet has been found very close our Solar system; on the 24th. August 2016 ESO announced the discovery around the star Proxima Centauri.

The location of Proxima Centauri in the southern skies

This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope.

Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani.

This discovery is special because the planet has been found around the closest star to us. Proxima Centauri lies 4.35 light-years away. It is to be found in the constellation Centaurus, but is sadly too far south for us in Britain to see. It is very faint as it is cool red star, it lies close to the brighter pair of stars known as Alpha Centauri AB. The planet has been designated Proxima b, and orbits its parent star every 11 days. It has a temperature suitable for liquid water to exist on its surface. This rocky world is a little more massive than the Earth and is the closest exoplanet to us — and it may also be the closest possible abode for life outside the Solar System.

Proxima was monitored as part of the Pale Red Dot campaign (https://palereddot.org/) during the first half of 2016. Proxima Centauri was regularly observed with the HARPS spectrograph on the ESO 3.6-metre telescope at La Silla in Chile and simultaneously monitored by other telescopes around the world.

The Pale Red Dot Campaign

Pale Red Dot was an international search for an Earth-like exoplanet around the closest star to us, Proxima Centauri. It used HARPS, attached to ESO’s 3.6-metre telescope at La Silla Observatory, as well as other telescopes around the world. It was one of the few outreach campaigns allowing the general public to witness the scientific process of data acquisition in modern observatories. The public could see how teams of astronomers with different specialities work together to collect, analyse and interpret data, which ultimately confirmed the presence of an Earth-like planet orbiting our nearest neighbour. The outreach campaign consisted of blog posts and social media updates on the Pale Red Dot Twitter account and using the hashtag #PaleRedDot. For more information visit the Pale Red Dot website: http://www.palereddot.org

Observations have revealed that Proxima Centauri is approaching Earth at about 5 kilometres per hour — normal human walking pace — and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri — only 5% of the Earth-Sun distance.

Proxima is a very different star to the Sun, not just in size and colour but also in activity; the conditions on the surface may be strongly affected by the ultraviolet and X-ray flares from the star — Proxima b receives 60 times more high-energy radiation than the Earth far more intense than the Earth experiences from the Sun. This made detecting a planet around it even harder! So far the rotation of the planet is unknown; it may be that one side always faces the star, whilst the other is in perpetual night. The border between these areas might be subject to intense winds – winds which may howl across the planet. This tidal locking of a body so that it faces the same direction is called synchronous rotation – the same reason we only see one side of the moon.

 

Artist's impression of the planet orbiting Proxima Centauri

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.

Credit:ESO/M. Kornmesser

Conditions on Proxima b may be very different to Earth and any life there will have had to adapt to its unique conditions, also the suitability of this kind of planet to support water and Earth-like life is a matter of intense debate. Major concerns that count against the presence of life are related to the closeness of the star. For example gravitational forces probably lock the same side of the planet in perpetual daylight, while the other side is in perpetual night. The planet’s atmosphere might also slowly be evaporating or have more complex chemistry than Earth’s due to stronger ultraviolet and X-ray radiation, especially during the first billion years of the star’s life. However, none of the arguments has been proven conclusively and they are unlikely to be settled without direct observational evidence and characterisation of the planet’s atmosphere.

Of course in the far distant future perhaps they will even become holiday destinations…!

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There is much to learn and discover, exciting times lie ahead!