Professor Don Pollacco, Department of Physics
Extra solar planets have come to prominence in the past decade as advanced techniques have begun to detect them in greater numbers. The discovery of Kepler-78b in October 2013 attracted media coverage worldwide, with some reports suggesting the planet had Earth-like properties. But what does that mean? Are we a step closer to finding another habitable environment outside of our solar system? Here, Professor Don Pollacco from the Department of Physics answers these questions.
Don, tell us what you do here at Warwick?
"I joined Warwick’s Department of Physics in September 2012 and I'm part of the Astronomy and Astrophysics Group. One of the main programmes we're involved in is WASP (a Wide Angle Search for Planets), a programme that finds planets as they move across the face of their star (the same as NASA's Kepler). Even in the era of Kepler, in terms of sheer numbers, WASP is still one of the most successful planet finding machines around and is responsible for well over a hundred confirmed planets. There are WASP instruments in the Canary Islands, South Africa and we’re building another facility at the moment in Chile. The purpose of this next generation transit survey will be to discover small rocky planets around much brighter stars than Kepler and these targets will be followed up by some of the European Southern Observatories largest telescopes in Chile."
I’ve also been involved in the follow up of Kepler candidates. Kepler, a NASA space mission has been surveying a small part of the northern sky looking for small planets and attracting a lot of attention in the process. In collaboration with Geneva Observatory, Harvard and three other UK universities, we’ve been using the HARPS-N (High Accuracy Radial Velocity Planet Searcher for the Northern Hemisphere) spectrograph which is based at the 3.5 m Italian telescope in the Canary Islands. We use this to look at some of the smallest planet candidates; looking for Earth analog systems amongst the Kepler candidates.
Ultimately one of the reasons I do this is to find out our place in the universe. What I mean by that is how unique is the solar system? This is a big question but it’s at the heart of what we do and it’s very exciting. When I started my career I would never have believed I would have a phase working in this area – it really is like Star Trek! I was brought up during the time of the Apollo moon landings and this was such an inspirational time I don’t think I had any choice but to do what I do."
So how do you discover a planet?
"Despite what you might see in the press, exoplanet discovery has not become routine. Planets shine through reflecting the light of their star and so are always billions of times fainter. As they are so distant from us, they are always very close to their star so we have the situation of an extremely faint object very close to an exceedingly bright object. Except in very special circumstances imaging the planet directly is currently impossible so we are forced to rely on indirect techniques.
The most successful methods involve either detecting their gravitation effect on their host star – the star wobbles as they both orbit their common centre of mass (analogous to two ice skaters holding hands and spinning round each other) or detecting a small dip in the brightness of the star as the planet moves across its face as it orbits (eclipse or transit technique). Using both these techniques together is extremely powerful as the transit can tell us how big a planet is and the size of the star’s wobble can tell us the mass of the planet. Hence we can determine the density of the planet.
Kepler and WASP both aim to detect transits. Both experiments have different advantages primary Kepler being above the Earth’s changing atmosphere can detect smaller eclipses while WASP is sensitive to much brighter stars and rarer types of planet. So in the case of Kepler 78b we already knew the planet was small and needed to measure its host star’s wobble to determine its mass. So that’s where HARPS North comes in.
HARPS North is a very special spectrograph. It is attached to the Italian 3.5m TNG telescope on the island of La Palma in the Canary Islands and lives in a giant thermos flask. This stops changes in ambient temperature (and pressure) effecting the data. It does this extremely well and is the most stable astrophysical spectrograph in the world. It can measure stellar wobbles with sufficient accuracy to detect a rocky planet around a sun-like star. Kepler 78b was a challenging object for HARPS North and needed a significant investment of time to show the wobble."
There was quite of bit of media coverage recently on Kepler-78 and Kepler-78b. What are they and what’s so special about them?
"Kepler-78 is the name of a star and Kepler-78b is the planet which orbits it. If there was another planet that would be called Kepler78c. Kepler-78b is interesting because it’s unlike any other planets that have been discovered. Although it’s about 20 per cent larger than Earth, with a diameter of 9,200 miles, and weighs almost twice as much, it’s the most Earth-like of planets that we have discovered so far. Other planets that appear to be Earth sized, (and there aren’t many) seem to be much more dense than Earth. In terms of mass, radius and mean density, Kepler-78b has similar properties to our planet.
The main reason 78 and 78b got a lot of media attention was because the Kepler space programme is led by NASA. They launched the space observatory in 2009 with the aim of discovering Earth-like planets orbiting other stars. NASA is very good at getting media coverage; in fact they are required to raise awareness of their work. But sometimes this means that some discoveries get more coverage and people read more into their relative significance than they should."
Why is this planet and star called Kepler and what relevance does the number 78 have?
"As mentioned, Kepler is the name of a NASA space observatory, essentially a telescope which is in space. It’s named after a sixteenth century astronomer, called Johannes Kepler. As for the number 78, there’s nothing special or mysterious about it. Each planet and star that’s discovered is given a number. So all it means is that Kepler-78 and Kepler78b were the seventy eighth star and planet to have been found as part of the NASA programme."
If Kepler-78b is Earth-like, does that mean it could be habitable?
"It’s true that Kepler-78b has Earth-like properties in terms of its mass, shape and density but that’s where the similarities end. The surface temperature of Kepler-78b is probably at least 2,000C higher than the hottest day on Earth.
Another example of how Kepler-78b is completely different to Earth is the amount of time it takes to orbit its star. Earth orbits the sun in 365 days, Venus, which is very hot because of its atmosphere, has an orbit of 225 days and Mercury - the closest planet to the Sun, has a temperature on one side of 800 degrees and is very cold on the other side, has a period of 88 days. Whereas Kepler-78b orbits its star in just eight hours, moving just above or in the atmosphere of the star. It’s quite different to Earth and couldn’t support life as we know it.
Will we hear any more about Kepler-78 and Kepler-78b?
It will go in the history books because it’s an Earth-like planet but I don’t think we’ll hear that much more about it. It will always be a challenge to find out more information about Kepler-78b because its star is quite faint (despite being one of the brightest Kepler stars!). Maybe when the next generation of extremely large telescopes are built there will be a new capability to study this object.
Ironically, as the WASP planets have host stars that are brighter, while they are bigger than Kepler planets, WASP planets are easier for follow up experiments such as planetary atmosphere studies."
So if the Kepler stars are too faint why bother to continue looking for them?
"Kepler came about because of a race between NASA and the European Space Agency (ESA) about fifteen years ago. The ESA planned to put a mission in space called Eddington and NASA had Kepler. But in 2003 the ESA dropped out because of budget cuts. It’s unfortunate because Eddington was capable of looking at much brighter stars compared to Kepler, and would have produced real transformational science. None the less, Kepler (along with a smaller French-led mission called CoRoT) is a pioneer, leading the way into this new science. Kepler results are the first large scale survey we have for small rocky planets – that has to be worth doing!
Kepler still has some 3,000 exoplanet candidates awaiting confirmation. This will be a pretty impossible task requiring an enormous effort from telescopes yet to be built and, I’m sure, many (maybe >90 per cent) beyond confirmation. However, the game is not over for the ESA yet as they are still considering another transit mission called PLATO – which builds on the Kepler results."
What does WASP do and how is it different to Kepler?
"So WASP and Kepler try to detect the same kind of signature – the dimming caused by an orbiting planet as it transits its host star’s disc. Of course we can’t see the disc but we can measure the dip in brightness. The bigger the planet (relative to its star), the deeper the dip.
Kepler is above the Earth’s atmosphere and can detect smaller dips but has a limited number of targets (150000 stars) and because it has a relatively small field of view, most of those stars are quite faint. WASP is sensitive to larger planets (Neptune sized and larger) but can see many millions of brighter stars. The important by-product of this is that WASP planets are much easier to confirm. The other thing of course is that being in space means Kepler has to be autonomous as it’s beyond any maintenance trip (and is currently not functioning at its optimum level). WASP on the other hand, while running robotically (i.e. on its own) is serviceable being located on sunny La Palma.
Image: One of the WASP survey instruments
The existing WASP instrument has been very successful but a few of us wondered could we do better? So we’ve secured funding to develop the next generation WASP survey instrument. We built a prototype, which was based on La Palma. For a long time I didn’t think anything would come of it, but after extensive work we found techniques to get the equipment to deliver extreme accuracy. The next generation model will improve our work and enable us to find much smaller planets, maybe even rocky planets.
Now the European Southern Observatory (ESO) have contacted us and asked if we’d be interested in putting the instrument in Chile. The ESO has the biggest telescopes in the southern hemisphere and they’re situated in the best place in the world in which to view the solar system. Hopefully we’ll start getting in some data next year, which will help to inform the ESO’s work. It’s going to be really cool and it’s all going on here at Warwick."
Do you think we will find Earth-like planet?
"Yep. If you mean a rocky planet in a habitable orbit around a star, we are probably pretty close to doing that now. However, if you mean an earth-analog (earth-like planet around sun-like star) then we’re probably 10-15 years off that. But definitely during my career."
Professor Don Pollacco joined Warwick’s Astronomy and Astrophysics Group in September 2012. A world leader in the search for planets around distant stars, Don played a crucial role in developing the Super WASP project, which has found more than a third of all known transiting planets and was awarded the Royal Astronomical Society Group Achievement Award in 2010.
Don recently took part in NASA’s Senior Review of its astrophysics space missions, and has been heavily involved in an advisory capacity with both the Science and Technology Facilities Council (STFC) and UK Space Agency (UKSpA) for many years. Don is also co-pi of the Next Generation Transit Survey (NGTS) and leads the exoplanet science programme of the proposed European Space Agency mission PLATO. These new missions are targeted at finding Earth like planets and enabling detailed studies of their atmospheres.