Sunday, 9 July 2017

Talk : Adventures in the Goldilocks Zone – The Search for Other Earths

For May’s Public Lecture Series talk, Professor Frazer Pearce joins us to discuss "Adventures in the Goldilocks Zone – The Search for Other Earths". @Gav Squires was there and has kindly written this guest post summarising the event, with some linkage added by NSB.

Jupiter is around a tenth the radius of the Sun. Earth is around a tenth the size of Jupiter. Jupiter is a thousandth the mass of our star while we are three hundredths the mass of Jupiter. An Astronomical Unit (AU) is the distance from the Earth to the Sun. Jupiter is 4 AU away from us and the solar system is around 50 AU in total.

Solar System Size Comparison

Eight billion years ago the Sun hadn't yet formed, it was a protostar. Close to the star, metals and minerals can condense into planets. As you get further out, you reach the "frost line" . Outside of this, low temperatures allow condensing planets to include things such as H2O, NH3 and CH4. The size of the star determines how far away this frost line is.

Prof Pearce, derived some of the key equations that describe how a planets orbit is related to its mass and speed, these are shown in the image below.

So if energy is added, the planet will move out from the star.

When the Millennium Bridge was built in London, it initially had problems with it swinging in sync with people who were walking across it. This is resonance. It is the reason that soldiers have to break step whenever they cross a bridge.

In our solar system, we can see that close to the Sun there were lots of rocky fragments, then out at the frost line Jupiter formed. After around 70,000 years, Jupiter started to migrate in towards the Sun, getting as close as 1.5AU. Saturn followed it and as it caught up, the two planets became locked in a resonant frequency. This prevented them from migrating in any further. When Jupiter was around 300,000 years old, it started to migrate outward from the Sun and reached its current position around 200,000 years later, thus ending the so called "Great Tack".

This explains a lot about the solar system including why we have so much water here on Earth - Jupiter had brought a lot of the frozen water from around the frost line with it. However, while it may feel like there is a lot of water on Earth, if you balled it all up, it would comfortably fit inside the US. There is still more water than would be expected though. Most comes from Jupiter but there is also some that came from comet bombardment. The grand tack is also the reason that Mars is so small. Mars is only 10% the mass of Earth but Jupiter gobbled up a lot of the stuff that should have been Mars.

Earth- Mars Size Comparison

There are two ways to look for an exo-planet. Firstly you can look for its transit - when the planet passes between a star and us. This will block out some of the light from the start. From this we can measure the period of the planet's orbit and so we can use Kepler's 3rd law, it's possible to work out how far away from the star the planet is. Then it's possible to work out the size of the planet by measuring how much light is blocked. The second way is through radial velocity variation. The planet and the star both orbit the centre of mass of the system. This means that the planet will cause the star to have a slight "wobble". This can be measured using the Doppler Effect. The bigger the planet, the more the star moves. Using these two methods together, you can work out the density of the planet, which tells us whether it is a rocky planet or not.

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

Bigger stars are better. There is a region around a star where water exists in a liquid state. Too near to the star and it boils off. Too far away and it freezes. This is known as the Goldilock's zone as it's not too hot, not too cold, but just right. The larger the star, the further away the Goldilock's zone will be. If a planet if 1 AU away from a small star, that would be no good as its water would be frozen.

The TRAPPIST-1 system, 40 light years away, contains 7 Earth-like planets. The star at the heart of the system is a red dwarf and is just a tenth the size of the Sun. The outermost planet is around 0.06 AU from the star and three of them are within the habitable zone. One could even potentially be a similar temperature to Earth. However, large flares from the star make life on the planets highly unlikely. The inner planets are tidally locked, like our moon, meaning that the same side always faces the star. In many ways the system is comparable to Jupiter and its moons. The planets of the TRAPPIST system are all in resonance - the inner most one orbits 12 times for each orbit of the outermost one.

Artist Impression of TRAPPIST -1 System

There are planets everywhere! Around 15% of systems contain Earth-like planets, 20% contain super-Earths and 20% contain mini-Neptunes. The next few years will see more missions launched [for example, TESS and CHEOPS] with the aim of discovering more exo-planets and specifically more Earth-like worlds. Earth itself would be too small to detect using its transit as it is too small to block out enough light. However, future detection techniques will allow the discovery of more planets similar to ours.

Professor Frazer Pearce

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:
Planets, Earth-Mars comparison, Kepler6b Transiting light level, TRAPPIST-1 system

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