Date: June 3, 2012

Title: Encore: Transiting Extra Solar Planets

Podcaster: The Ordinary Guy from the Brains Matter podcast

Organization: Brains Matter –


This Podcast originally aired on October 31, 2010

Description: The number of extra solar planets discovered keeps increasing every time we hear about them. How do astronomers work out they are there? The original method used was called the Radial Velocity method. These days, a lot of work is being done using the Transiting Method. I talk to Dr Rosemary Mardling from Monash University about the Transiting method, and what this has meant to the discovery of extra solar planets.

Bio: The Brains Matter podcast has been producing and communicating science stories and interviews since September 2006. The show is based out of Melbourne, Australia, and takes an everyday person’s perspective of science in easy-to-understand language.

Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.”

Ordinary Guy: Hello everyone, and welcome to the October 3rd episode of 365 Days of Astronomy. I’m the Ordinary Guy, from the Brains Matter podcast. You can hear shows on science, curiosities, and general knowledge at We’ve all heard about extra solar planets, and the number discovered just keeps on going up and up every time there’s news about them. I spoke to Dr Rosemary Mardling, an extra solar planets expert, who recently came back from a conference in Europe discussing the topic.

OG: I think the last time we spoke, there were probably about 300 extra solar planets that had been discovered and it’s a lot more than that now isn’t it?

Dr Rosemary Mardling: Yeah, well I think that the published ones is something like 470; I haven’t looked at changes every day but I do know from my secret contacts that there are well over 500 now. They’re getting a bit behind with publishing their results because there are just so many of them now. And one of the most amazing things that they’re finding is that there are many many low mass planets – many more than the big guys like Jupiter. There are many planets like Neptune; well of the mass of Neptune – we don’t know if they’re actually LIKE Neptune. Well, it’s got a solid core, we believe, we’re not even completely sure about that actually; and an atmosphere. Perhaps some of these extra solar planets that are similar mass are completely solid, or even water worlds, we don’t know.

OG: Even just a year ago – well, a year or two years ago, we were talking about just seeing what they called “Hot Jupiters”, and couldn’t see anything smaller, because we didn’t have the capability to, or the technology to detect things like that.

RM: That’s right.

OG: But now we are, you are saying?

RM: That’s right. Well, there’s an instrument that – the Swiss have designed an instrument that is called the Harps Spectrograph, well it’s an ESA instrument, at the European Southern Observatory actually, in Chile, and it’s a spectrograph that’s been put in vacuum and this has allowed them to have much higher accuracy than thought possible before. So they’re able to, well, we talked before about how they discovered most of these extra solar planets using the radial velocity method, so they measure the wobble of the star, the speed of the wobble of the star due to the presence of a planet orbiting it. And now they can measure wobble speeds down to one metre a second! I mean, that’s just a casual stroll. And in fact they will be able to – they ARE already actually, able to measure even less.

OG: Wow

RM: Even smaller speeds. There are all sorts of compilations that go with that and limitations that they thought they wouldn’t be able to overcome, especially the fact that the motions of the star itself, I mean the gas – the speeds of those motions are higher than a metre a second, and it was thought that would just sort of wash out any small signal…

OG: That would be the limit of it..

RM: Yeah… but they know how to take that into account now, and they can actually – if they have enough observations – they need lots of data, they have to keep going back and looking at the same stars over and over again – if they have enough observations, they eventually they can so called “pull” the signal out of the data. It ‘appears’ the longer you look at it, if you know how to model, how to take into account the stars motions and the magnetic – these stars have magnetic cycles like our sun does, and that can show up and look like a planet. They know how to take that into account.

OG: It’s almost like looking at one of those 3D pictures isn’t it – you stare at it and if you look at it the right way, then the information comes out.

RM: Ah yeah! That’s right. If you stare at it for LONG enough, that’s the analogy, yeah. That’s right.

OG: So you’ve just come back from Europe, haven’t you? What were you doing over there, and what did you learn?

RM: Well, one of the the big hot things of the last few years is this. I mentioned radial velocity measurements, but transiting planets…

OG: That’s what the Kepler instrument is doing, isn’t it?

RM: Exactly, yup. And also some ground-based experiments. Various experiments around the world to … two in particular, the WASP Consortium and the HAT Consortium have discovered lots of transiting planets, so a transiting planet … a transiting system is one in which the planets – the planet-star orbit happens to be ‘edge on’. We’re looking at it side on, so we can see the effect of the planet moving across the face of the star; it’s an eclipse.

OG: So there’s a dimming and brightening of the …

RM: Yeah.. so it blocks out some of the light, and so we can plot that, we can measure that dimming and from that, we can work out or measure the time it takes for the planet to move across. We can work out how much we can measure, how much light is blocked out so we can actually work out the radius of the planet if we know, if we can estimate the radius of the star. Doing this we can compare… well, we can estimate the – we can actually work out the mass of the planet and we can then work out the density of the planet if you know the radius and the mass. Then we can compare that to Jupiter, if it’s a Jupiter kind of planet, and we can say “hey this one’s much bigger than Jupiter, what’s going on?” Or we might have a plane that’s much smaller than we expected and we can say “well this thing is really dense” etc. etc. So having said – I always go off on tangents, I’ll come back to your question – the conference I’ve just been at, in the south of France, was on transiting planets and dynamics, and that’s a big reason I was invited to speak there because I’m a dynamicist. With all this information coming from transits, as well as radial velocity surveys we can now say a lot more about the orbits, and even things like the interior structure of planets that are close to their stars, and all sorts of things that we could’t say before. Putting these two kinds of observations together. And you mentioned the Kepler satellite before. Kepler is a space mission to measure transits and their big aim is to catch Earth-like planets in the act of transiting their stars. And they’ve come back with all sorts of amazing surprises. They’re keeping the best still under their hats at the moment, so they can study them a bit more before they release the results. One of the most exciting results for us, for me as a dynamicist, is seeing multiple planets – multiple transiting planets – so that means they see more than one planet transit the same star…

OG: Around it…

RM: … and if you think about it, well, there are different geometries you can have but the most likely one is that they’re in the same plane and they have to be so precisely in the same plane to both transit that this is like, this is mind blowing. This is really beautiful and it’s telling us something about how these planets form and, well, nature’s always full of surprises and and it’s the most exciting time to be a scientist, is when nature comes and you know, pokes fun at us really… and we thought we understood how something works and then it comes and shows us something completely different. One of the exciting results announced at that conference was the discovery of a seven-planet system.

OG: Wow, so that’s almost like us!

RM: So these radial velocity guys have got their art down to … you know, so fine, they’re doing such a good job that they can actually pull out the signal of seven planets, and they’re low mass planets actually, they’re Neptune-y kind of planets – well there’s a Saturn in there as well. And they’re quite close. In fact they’re quite close to each other and they’re close to the star. In fact their longest orbital period is 90 days.

OG: Wow.

RM: So they’re all packed in inside Mercury’s orbit. Seven of them! And happily sitting there for billions of years, so the system is stable, and this is just the tip of the iceberg.

OG: This has got implications for our understanding of how our solar system was formed, doesn’t it?

RM: Exactly. That’s right, it does. Well it just shows that the diversity of nature and that the solar system is just one special case and actually one of the big questions to answer is how special is the solar system?

OG: Up until now we’ve had a sample size of one.

RM: One! That’s right. Are there lots and lots of Solar System type systems out there? That is, where you have the gas giants orbiting far from the star probably around where they formed, and terrestrial earth-like Earths and Venuses and things on the inside, and some Neptunes on the outside. Is that typical, or is that a rare kind of system? Most of the planets we’ve discovered so far are on elliptical orbits – significantly elliptical orbits. The solar system planets are almost circular orbits, and we’re finding lots of Neptuney kind of systems very close to their stars – there are lots of these things. And as I mentioned before, they seem to be more common than systems with Hot Jupiters, so you know, it’s just the beginning of the story. It’s really good to be in a brand new field like this, well it’s new in the sense that the first one of these kinds of systems was discovered only fifteen years ago and there’s so much to do, so much to think about.

OG: Sounds all very exciting, so thank you for that update Dr Rosemary Mardling.

RM: My pleasure!

OG: And thanks for listening to todays 365 Days of Astronomy. Check out for more interesting stuff. Bye for now!
End of podcast:

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