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

Talk : Arachnoglobia - what makes a spider fly?

For April's Cafe Sci talk, Dr Sara Goodacre, who is an Assistant Professor at the Faculty of Medicine & Health Sciences at the University of Nottingham (and runs the UoN Spider Lab) gave a talk entitled "Arachnoglobia - what makes a spider fly?"

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

Spiders eat pests. If we could work out how to control their populations then we could end up with a more environmentally friendly world.

The spider family is 400million years old. Spiders have adapted to their world in many ways. Why, and how do these difference help the spider to adapt? In a single species of spider, why does more than type persist? The adaptations that they have are all relative to each other. Even in a single species, different morphs can exist in different parts of the world. To our eyes these morphs look different but they can appear as different again under UV light. In fact, Crab spiders pretend to be nectar to catch bees. This effect can only be seen in UV but it works because bees see in UV.

Crab Spider about to spoil a bees day

In some cases, spiders are finding the same solutions to survival - some can have the same shape for a completely different genus. Co-operation can also solve some problems. Some spiders live in groups and they live in perfect harmony with each other. We've seen this in 9 species of spider, which is impressive but there are 40,000 types of spiders in the world. Those that do live in close family groups end up inbreeding. However, after 9-10million years of this, there appear to be no adverse effects. There is also a huge female bias when it comes to spiders in group living conditions.

Social Spiders

Being able to move long distances also helps. Lingphiids are common in disturbed, agricultural environments. Many are capable of dispersing long distances by spinning out a line of silk and using it to take off. This is how they can end up on a farmer's field even after they have sprayed pesticide. You can find around 100 spiders in the space the size of an average table and they eat 25 times their body weight in pests every week. Spiders are 1000 times more impacted by insecticide than whatever you're trying to kill. How many of these flying spiders carry resistance to pesticides with them?

At any time, 40-60% of spiders will attempt to take off. This achieves optimum balance between rates of extinction and recolonization. After a new island emerges from the sea, spiders are often the first animals on it and birds in places like Svalbard partially rely on flying spiders arriving as a source of food. Prior to take-off, 1.5m of silky sail is spun out. The spider cannot control how high they go or where they end up. Spiders can't propel their silk out, it has to be reeled out instead. Most species of spider eventually get too big to be able to fly. Money spiders can fly throughout their lives though. Spiders can fly up to 70km in just 10 hours.

Why is dispersal so important? Professor Godfrey Hewitt said "Global climate has fluctuated greatly during the past three million years, leading to the recent major ices ages. An inescapable consequence for most living organisms is great changes in their distribution. Such range changes can be expected to have genetic consequences."

Single filament of synthetic spider silk
What drives the different dispersal strategies? What is the relationship between dispersal, gene flow and local adaptation?

Risk avoidance on the part of the spider is not the only factor determining when it flies. It has been discovered that spiders carry particular bacterial infections that are known to influence the biology of other hosts. For example, in butterflies, this Wolbachia bacteria results in a female biased sex ratio, meaning that the females have to compete for males. In woodlice, genetic males develop as females but other males prefer real females over feminised males. In other creatures, infected females aggregate offspring, thereby promoting sibling mating and inbreeding. The bacteria affects higher brain function.

Example of Spider Adaptations

How can we determine the effects of bacterial infection in spiders? We can cure the spiders with antibiotics and then see what happens to the behaviour of the "cured" spiders. Of course this isn't as straightforward as it sounds - what is the right dose of antibiotic for something as small as a spider? It turns out that spiders are more likely to fly if they have been cured. So, the bacteria see to influence whether or not a spider will fly. Around 2/3 of invertebrate creepy crawlies have this bacteria.

Spiders that can fly are also good sailors. This is how they can travel much further than we initially thought. Some spiders can even stand on water. These are a subset of the sailors, who are a subset of the flyers. The spiders that can fly but can't sail might be in trouble if they land on water. There are even some spiders that can survive submerged for up to a week. However, many spiders can drown in a raindrop.

Spider Distribution Map at BAS

We then move onto a slightly more general chat about spiders. There are very few spiders that are actually dangerous to humans. There is Brazilian wandering spider that uses neurotoxin but most spiders don't have that kind of venom. Very few cases globally of a long term harm from a Sydney funnel web. There have only been two reports of "Fake Widow" attacks, despite all the coverage that they have received in the press. One was a sore arm for an afternoon in Worthing in 1991. One was a sore leg in south France in 2003. However, you are still better not touching spiders abroad but there are no harmful spiders in the UK.

Many people think that conkers can be used to keep spiders away but they do nothing. If anything a spider just sees a conker as something else to hide under. Most spiders can mate twice in quick succession. Sometimes a male spider is half-eaten, mates again and then she eats the rest of him. The hairs on a spider's foot that let it climb glass are thinner than a human chromosome.

In conclusion, natural selection shapes traits such as ability to survive encounters with water and aerial dispersal tendency. A small proportion of individuals is capable of moving over distances far greater than previously imagined because aerial dispersers are able to survive encounters with water.

Dr Sara Goodacre

Café Sci returns to The Vat & Fiddle after its summer hiatus on the 9th of October where Mike Merrifield will talk about Exoplanets. For more information, check out the Café Sci MeetUp site:

Image Sources
Crab Spider, Social Spider, Spider silk comparison, Silk strand