Sunday, 22 October 2017

Talk : Exoplanets

Café Sci returns after its summer hiatus with Michael Merrifield talking all about exoplanets.

@Gav Squires was there and has kindly written this guest post summarising the event, with some linkage added by NSB.

Exoplanets are defined as those planets that are outside our solar system and we now know of around 3,500 of them that are orbiting various stars.

In the middle of the 18th century, Immanuel Kant gave us the modern view of how the galaxy works based on observations of the solar system. We'd extrapolated our understanding out and applied it to the whole universe and we thought that we knew how everything worked. For example, in the solar system, all of the big planets are outside the ice line whereas those planets inside are smaller. Using ice is one of the easiest ways to make things stick together and we assumed that we would see the same big/small planet split in other star systems.

Prof Merrifield, via Gav Squires

The first exoplanet, the catchily titled HD114762b, was discovered in 1989 by David Latham. It was detected by measuring the Doppler shift as the star was effected by the gravity of the planet. Since it was thought that large planets (HD114762b is about 5-10 times the size of Jupiter) wouldn't be that close to a star and so it was originally misidentified as a brown dwarf star. A pulsar is the end state of a star - as they spin around, they can be used like a clock and are actually more accurate than atomic clocks here on Earth. In 1992, three planets were discovered orbiting a pulsar.

A quarter of the exoplanets that have been discovered have been found using this Doppler shift method, looking for the effect of the planet's gravity on the star that it orbits. Most exoplanets are discovered using the occultation method though. As a planet passes between us and its star it blocks out light from the star. We need a fair bit of luck for this method to work though as the orbit needs to be edge on for us to notice the planet's transit. This technique will give the radius of the planet and the radial velocity will give the mass and so we can calculate the density (as an side, Saturn is less dense than water). It is also possible to tell about the atmosphere of exoplanets and how much light is reflected from their surface.

Kepler6B photometry - showing light from star being blocked as planet passes in front

Occultation can even help to discover moons around exoplanets. In the summer, Kepler-1625b1 was tentatively announced as the first ever exomoon - it seems to be a Neptune-sized moon orbiting a Jupiter-sized planet. Another technique for discovering exoplanets is micro lensing - a star with a planet produces a different effect compared to a star without one. This is better at detecting planets that are further away from the star but it is a one-shot deal as it won’t line-up again.[a list of exoplanet detection methods can be found here]

Exoplanet systems are incredibly common, even our nearest star, Proxima Centauri, which is 4 light years away, has an Earth-sized planet orbiting it. The planet is at 0.05AU from Proxima Centauri and orbits once every 11 days and it could be tidally locked - the same side faces the star in the same way that one side of the moon always faces Earth. Proxima Centauri is a weak star but the planet still gets around 70% of the sunlight that Earth gets. This means that it is around -39oC but there could still be liquid water there if there is a greenhouse effect. However, there is unlikely to be any life as Proxima Centauri is a flare star - it regularly emits massive x-ray stellar flares, irradiating the planet.

Professor Stephen Hawking is involved with a project called Star Shot that would like to send a satellite to visit the planet around Proxima Centauri. By firing a powerful laser at a spacecraft, the size of a postage stamp it would be possible to speed it up to 20% the speed of light so it would arrive in just 20 years. Of course, there is still the issue of creating a satellite that small and many such satellites would need to be sent to cover the possibility that they could crash or fail. Once fired off, it wouldn't be possible to slow them down either so each satellite would only be able to make one fly past of the planet.

Based on existing technology, a more realistic idea may be using a solar sail - a satellite the size of a bar of soap would require a sail the size of 10 football fields. Light from the sun would speed it up, although it would take 80 years to get there. We are already able to make a satellite around that size and we would be able to use Alpha Centauri to slow it down in order to actually take some photographs.

NASA illustration of the unlit side of a half-kilometre solar sail
TRAPPIST is the really contrived acronym for a pair of Belgian telescopes that discovered the TRAPPIST-1 system, which features 7 exoplanets. The star is the size of Jupiter and the planets are orbiting at the same distance as the moons of Jupiter. 3 of the planets are in the so-called habitable zone and all 7 of them are in resonance with each other, which is what has actually led to the stability of the system. The SETI people have been focussing on this system but haven't heard anything yet.

Artist Impression of TRAPPIST -1 System

The term "Hot Jupiter" was coined to describe these massive planets that are close to their stars and their discovery has meant that we have had to rethink what we know about the creation of stellar systems. We are now starting to see the discs around stars - planets from in the gaps in these discs so if we can see these gaps, it implies that planets are starting to form and we can hopefully start to learn more about how planetary systems form.

The European Extremely Large Telescope is currently being built in Chile and is set to be completed in 2025. Its main mirror is 39 metres in diameter and it is proof that we can now do things on the ground that we couldn't even conceive of 10-15 years ago - not all telescopes need to be in space. The EELT will have such sharp images that we'll actually be able to see the planets themselves. We will even be able to tell which wavelengths are absorbed by their atmospheres - if we detect ozone, it would be proof that there is life on a planet.

There is a huge bias in the planets that we have been detecting since it is easier to find big planets - we've just been discovering the easy ones. We are starting to discover some oddities though - it seemed like one start, Tabby's Star, might even have a Dyson Sphere around it. While the changes in brightness emanating from it were relatively consistent with what was expected from a Dyson Sphere but the latest results show that UV & IR absorptions are different, meaning that it looks like it’s just naturally occurring dust.

Café Sci will return to The Vat & Fiddle on the 13th of November at 20:00 where David Nicholson-Cole from the University of Nottingham will talk on The Skyscraper - From mid-20th century to 2030. For more information check out the Café Sci MeetUp page:

Related Content:
Fee- An Autobiography
Curiosity, Twitter and the British Connection
Interview with Prof Aragon-Salamanca
Interview with Prof Chris Lintott
Some background to the Space Shuttle
Lecture by Chris Lintott on 2011 Astronomy highlights

Image Sources:
Kepler6b Transiting light level, TRAPPIST-1 system Solar Sail

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