Date: September 16, 2011

Title: Can Habitable Planets Be Found Orbiting White Dwarf Stars?: Q & A with Eric Agol

Podcaster: Chris Lindsay

Links: For more information about Dr. Eric Agol, please see

For more information about the Bruce Gary PAWM (Pro-Am White Dwarf Monitoring) Project, please see

For more information about the Ann Arbor Science & Skeptics, please see

For more information about the Critical Wit Podcast, please see

Description: This episode consists of an interview with Eric Agol, associate professor of astronomy at the University of Washington. Dr. Agol has recently written a speculative paper stating that habitable planets might be found orbiting white dwarf stars, and why searching for them might be a worthwhile effort. This has led to a pilot project called PAWM (Pro-Am White Dwarf Monitoring).

Bios: Dr. Eric Agol is an associate professor of astronomy at the University of Washington. Eric has been working on the theoretical development of new techniques for detecting planets around other stars using novel techniques: planetary transits, timing of transits, corona-graphic imaging, and radial velocity searches with fixed-delay interferometry.

Chris Lindsay is the organizer of the Ann Arbor Science & Skeptics, and host of the Critical Wit podcast, a podcast about science, literature, and the arts.

Sponsors: This episode of “365 Days of Astronomy” is sponsored by the Ann Arbor Science & Skeptics group in Ann Arbor, Michigan. You can find them at

This episode of “365 Days Of Astronomy” has also been brought to you by Greg Dorais, just because it’s a really cool podcast.


Hello. Thank you for listening to 365 Days of Astronomy. I’m Chris Lindsay, the organizer of the Ann Arbor Science and Skeptics and host of Critical Wit, a short weekly podcast about science, literature, and the arts.

The Pro-Am White Dwarf Monitoring project is a pilot survey consisting of professional and amateur astronomers looking at white dwarf stars to see if they offer indications of possible orbiting planets. I spoke with Dr. Eric Agol, an associate professor of astronomy from the University of Washington to explain why searching for habitable planets around white dwarf stars is a worthwhile endeavor.

Chris Lindsay: Eric, you have speculated that it might be a worthwhile effort to look for habitable planets orbiting white dwarf stars. First, what is a white dwarf star? And second, why should we be looking there for habitable planets?

Eric Agol: The first question, why white dwarf star…these are stars, like our sun, that will eventually burn through their nuclear fuel – and so they are basically just radiating energy into space and no longer producing energy, at least not by nuclear sources. And so they cool off with time. And they are the remnants of the cores of stars which compress to become extremely dense, so that a star that’s – say half as massive as our sun – will occupy a volume that’s slightly larger than the size of the Earth. So they are extremely high density. So, white dwarf stars are about as common as stars like our sun. But due to their small size and – as they cool – their small luminosity, they can be very, very hard to find. So astronomers actually, in active area of research are trying to find white dwarf stars. Now, the reason why I thought about looking for um, so-called habitable planets around white dwarf stars is that the, as the white dwarfs cool, the distance at which a planet like our Earth if it were to orbit a white dwarf star, would be at the same temperature as our Earth would – be very, very close to that white dwarf star. So, essentially it is like being around a campfire. The smaller the campfire, the closer you have to huddle around it to maintain your temperature. The same thing occurs for a planet around a white dwarf. For that planet to have a similar temperature as the Earth, it has to snuggle up close to the star. And as a planet gets closer to the star, there is an increased probability that the geometrical alignment of the planet’s orbit will be such that it can pass in front of the star, and when it does so, it can eclipse the white dwarf star. Now, because white dwarfs are similar in size to the Earth, if our Earth were to orbit close to a white dwarf star, and were to pass in front of that white dwarf star – as seen from the perspective of an outside observer – they would see a large amount of light from the white dwarf star blocked by the planet. And so it would be very easy to detect the eclipse of the white dwarf by the planet. And this is what led to the calculation I did, where the habitable zone would be, and then how you can go out and look for these planets, using the eclipses of these stars.

Chris: So, then we’re really talking about using one of the more popular methods in finding exoplanets, in general, and some of the exoplanets that have been found, have been done this way, where a planet is orbiting its star and when it passes between the star and our observation, then the star’s brightness dims. Is that right?

Eric: That’s right, yeah. An interesting thing I found is that if you look at stars like our sun, the habitable zones are fairly far away from the star in relative to the size of the star. And so for our sun, the habitable zone of the Earth is about 200 times further away than the size of our star, the Sun. What that means is that the probability of the Earth going in front of the sun has seen by an observer in another solar system, they [the observer] would have a 1/200 chance of seeing the Earth’s transit, or a partial eclipse of the Sun. And if they were to witness that, they would see a very, very, small dip, about one part in 10,000, because the Earth is about 10,000 times smaller than the Sun as far as area, and that’s a very, very small enough change in the brightness of the Sun – it’s difficult to detect. The other thing is because the Earth orbits at one astronomical unit (1AU) from the Sun, the period of our orbit is one year by definition, and so you’d have to wait a year for the planet to go around the star and eclipse it again. Now typical white dwarfs, the most common white dwarfs have a luminosity, the amount of energy that put out every second, or the wattage they put out every second, is about 1/10,000th of the luminosity of our Sun. And so, a planet has to orbit a hundred times closer in order to receive the same amount of energy as the Earth receives from the sun – this is how the habitable zone is defined – because the planet has to be that much closer to the white dwarf, the orbital period is much, much shorter. It’s something like twelve hours as compared to a year for our Earth to orbit around our Sun. And so there’s a much shorter amount of time you have to wait for the planet to orbit in front of the [white dwarf] star. You can see many, many repeated events. In addition, as I mentioned, because the Earth is a similar size of the white dwarf, it can nearly, completely, eclipse the white dwarf. So instead of one tiny little dip as the Earth goes in front of the sun, and trying to be detected by alien civilizations, the Earth would go in front of a white dwarf, you would see a dip of about 50% and so that’s much, much easier to detect with a telescope. The disadvantage though is that the duration of the eclipse would only last for a couple of minutes, so you have to take data quickly, in order to catch the eclipse event as the planet goes in front of the star. So, because of the short period of the orbit, and because the large depth of the eclipse, in some ways finding Earth-like planets around white dwarfs is much, much easier than finding Earth-like planets around Sun-like stars.

Chris: Now do you see any complications, such as a planet that is ‘that’ close to the star would most likely be either tidally-locked to the star, or it would have been baked by the star’s red-giant phase before it became a white dwarf star?

Eric: Yes, and that’s exactly why no one has written a paper on this before I have this year (laughs). Because, before a white dwarf is formed, stars go through a phase in which they are burning through their fuel very, very rapidly, so they become extremely luminous and they puff up to giant sizes, so a star like our sun can expand in its size to about the size of our [Earth’s] orbit. So it’ll basically obliterate everything in its path, so any planets orbiting close to that star, would be enveloped by the star. That star then blows off most of its mass into outer space, and the remaining core shrinks to become a white dwarf. And so nobody really expects there to be planets orbiting white dwarfs, and so nobody has really looked into this idea which is why, it remains unexplored until I wrote a paper on this. And the reason, I was motivated to write a paper on this is both the reason that, because it’s so easy to look for Earth-temperature, Earth-size planets around white dwarfs – in some ways, you might as well just do it because it’s not that – not too hard – it’s hard in some ways, which I’ll come to that…So, and actually, Freeman Dyson, a physicist of the Institute for Advanced Study at Princeton has a quote which he says that, “even if it is something that’s completely unexpected in nature, if it’s easy to find, then you may as well go out and look for it.” That’s kind of the nature of this paper. In addition, there are some hints that there may be ways in which planets can survive the red giant phase, the bloated, expanded stellar phase, a planet further out may be able to survive and then later fall inward or somehow be kicked inward so that it orbits closer to the white dwarf star, or possibly, there could be a second generation of planets that forms around white dwarf stars, that can be then much, much closer to the white dwarf. And in particular, we see planets orbiting Sun-like stars that are very, very close. We call these ‘hot Jupiters,’ they’re a similar size to Jupiter, but they orbit their stars in just a few days. And so, if similar planets were to form or re-form around a white dwarf star, they can be-ranging in different sizes, but if some of them that formed are the size of the Earth, then these are the types of planets you can target to look for Earth-temperature, Earth-size planets. In addition, white dwarf stars, as I mentioned, aren’t producing much energy, they’re not nuclear burning, they actually do release some energy due to crystallization of their interiors and some gravitational settling, which releases gravitational energy, which then gets converted into heat. So the remaining nuclear energy, plus this crystallization energy and the gravitational settling energy causes the white dwarf to shine for a very long time. And it turns out that a planet orbiting a white dwarf at a distance that’s a hundred times closer than the Earth is to the sun, the time that it would spend in between the temperatures of Mars and Venus, is about 8 billion years. So, even though the planet [correction: star] is no longer burning nuclear energy, the rate of its cooling is so slow, that the white dwarf, the temperature will change very, very gradually and the planet should spend a long time at very nearly the same temperature. And so this is an attractive feature, that planets orbit close to white dwarfs, should have very stable temperatures, for a long period of time. But as you mentioned, when the white dwarf is very young, [these] young white dwarfs turn out to be very, very hot. That’s why they are called white dwarfs, actually. And so, if a planet were to orbit a white dwarf soon after the white dwarf is formed, the white dwarf is still emitting a fair bit of energy, and so it can evaporate the atmosphere of that planet. And so, for these planets to be similar to our Earth, you know similar atmosphere, and similar oceans, we’d have to either re-form the atmosphere and the oceans later on. And there are several possibilities for doing this, one is by outgassing from the planet, another possibility is by having comets that survived from further away, and that retained their water, some of them can fall inwards and hit the planet and deposit some of that water, or maybe the planet can just form late enough, where the white dwarf has cooled enough, such that it wouldn’t evaporate the atmosphere or oceans of the planet. So this is really a speculative idea and it’s not clear that even if these planets are in the so-called habitable zone, it’s not clear that these planets would be habitable. It’s a necessary, but not necessarily a sufficient condition for the planet to orbit at the right distance and right heat for liquid water temperature, it does not necessarily mean that there will be the water there.

Chris: So talk a little bit about this project that is starting up, and going to run through September, the PAWM project. The Pro-Am White Dwarf Monitoring.

Eric: Yeah, this is…was started by an amateur astronomer, Bruce Gary, who hails from Arizona, who has done a lot of follow-up observations of stars that have known transiting planets. And after hearing about my paper, he decided to organize a world-wide effort to monitor white dwarfs. Now, I said finding planets around white dwarfs would be easy, but easy is a relative term though. And that it would take, if every white dwarf had an Earth-temperature, Earth-sized planet orbiting it, it would still take observations of a hundred white dwarfs to have the right alignment between the orbit of the planet and the star to cause transits to occur. So, you still need to observe, something like a hundred white dwarfs to find one that has a habitable planet, assuming that all hundred of them have one. And so, um, so now let’s say only one in hundred white dwarfs have an Earth-like planet orbiting at the right Earth-like temperature distance, then that’s going to require something like ten thousand white dwarfs. Um, which is a large number of white dwarfs to survey, and if you stare at each them for an order of a day to have a good probability of having a planet pass in front of the star. And so, ten thousand times a day, you’re talking ten thousand days of observations, which is getting to be quite a bit amount of time. But you can imagine, if you had enough observers and telescopes around the world, you can just train each of them at a different white dwarf, then that would reduce the amount of time, just scaled versus the number of telescopes. And so, um, what Bruce is organizing is a pilot survey that’s going to occur during September, so far about thirty-five astronomers – both amateur and professional astronomers – have signed up from around the world. There’s actually a map on the website that shows the distribution of telescopes around the world. And the goal is to just do a kind of a pilot survey. I’m hoping that we’ll observe, something like 30 white dwarfs, so it’s not enough to, you know, to put good constraints on the presence of Earth-like planets, but we could actually have a higher probability if there were Jupiter-size planets – then the probability goes to about 5%, rather than 1%. So with 30 stars, we’ll start to put constraints on the presence of larger planets. And the goal is to have enough coverage, from all these different telescopes, of enough white dwarfs that we might have a chance – it’ll still be a small chance, we might have a chance of detecting transits of planets around those white dwarfs. And even if we don’t see anything, at least learn about what quality data can be taken in conjunction with the size of the telescope, the location of the Earth, etc., so that maybe a future, larger, survey can be carried out with a larger number of observers. So, that’s the goal of the survey, and hopefully, we’ll find other interesting and unexpected things, as most astronomers have found serendipity is usually your friend in astronomy, and usually you find things that you didn’t expect to find, when you go out and look at stars in a way you’ve never looked at before. So the novelty of this will be just staring at a large number of stars continuously and actually isn’t done too often in astronomy, so it’ll be interesting to see what comes out of this.

Chris: Well, thank you very much for your time, Eric, to chat with me today on 365 Days of Astronomy.

Eric: Thanks, Chris.

End of podcast:

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