Date: November 2, 2010

Title: The “Goldilocks” Planet and Finding Planets

Podcaster: Rob Knop

Organization: Quest University Canada

Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php/MICA_Events

Description: If you’ve been paying attention to the buzz in the astronomy world, or even to news reports about astronomy, in the last month, you’re aware of the “Goldilocks” planet, Gliese 581g. Today, I want to talk a little bit about how we found this planet, and how extrasolar planets in general are found.

Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored anonymously and dedicated to the memory of Annie Cameron, at the time of NASA EPOXI flyby of Comet 103P/Hartley 0.0.155 AU above Tryphena, Great Barrier Island, New Zealand, located between Betelgeuse and Procyon on the edge of Canis Minor 4 November 2010.

Transcript:
The “Goldilocks” Planet and Finding Planets

This is Professor Rob Knop, tutor of physical science at Quest University Canada in Squamish, British Columbia.

If you’ve been paying attention to the buzz in the astronomy world, or even to news reports about astronomy, in the last month, you’re aware of the “Goldilocks” planet, Gliese 581g. Indeed, I recommend that you listen to the “365 Days of Astronomy” podcast from October 6, in which Pamela Gay and Fraser Cain discuss the properties of this planet, which are rather harsher than you might have deduced from the media stories about it, and the prospects for finding life there. Today, I want to talk a little bit about how we found this planet, and how extrasolar planets in general are found.

There are several techniques that astronomers have used to find planets outside our own Solar System. What would seem like the most obvious technique, taking very long exposure images to look for the dim images of far away planets, is actually not the technique that we’ve used to find most of the planets we know about. These planets are so far away, and usually close enough to their star, that even if they are bright enough to see, they are lost in the glare of their star. We have imaged a few planets, but the vast majority of planets we’ve found have been found through indirect methods.

The technique that found Gliese 581g is the one that found the first planets outside our Solar System orbiting the Sun, and to date has been one of the most fruitful techniques for finding extrasolar planets. It takes advantage of the gravitational attraction of the star to the planet. Yes, you heard that right. Normally you think about the gravity of the star holding the planets in their orbit. It is the gravity of the Sun on the Earth that makes the Earth orbit the Sun. The Sun is so much more massive than the Earth, that it is the Earth that goes around the Sun, and not the other way around, no matter what our ancient ancestors may have believed. Likewise, the Sun is much more massive than Jupiter, the largest planet in our own Solar System, that Jupiter orbits the Sun.

However, there is in fact a gravitational pull on the Sun due to the mass of Jupiter. In fact, it turns out that the gravitational pull on the Sun due to Jupiter is exactly the same as the gravitational pull on Jupiter due to the Sun. It’s just that because the Sun is so much more massive, that amount of gravitational pull doesn’t do nearly as much to the Sun as it does to Jupiter. But it doesn’t do nothing. We say that Jupiter orbits the Sun, but it would be more precise to say that Jupiter and the Sun both orbit their common center of mass. That center of mass is extremely close to the Sun– indeed, that center of mass is barely outside the radius of the Sun. So, while Jupiter orbits in a wide circle about the Sun, the Sun just sort of wobbles about. In the 12 years it takes for Jupiter to make one circuit in its orbit, the Sun makes one complete circle in its wobble.

If the wobble is so small, how is it that we’re able to measure it for other stars? The wobble is small enough that we can’t actually see the star wobbling back and forth. However, we are able to very precisely measure the speed at which an astronomical object is moving towards us, or away from us. This takes advantage of the Doppler Effect. You are probably familiar with the Doppler effect in sound. When a car goes by you, you hear it make a rrrraAAAAAARRRRrrrrwww sound. When it’s coming towards you– rrrrrr– you hear it at a higher pitch than after it’s passed and it’s moving away from you — arrrrrwwww. The higher pitch is a shorter wavelength of sound, and the lower pitch is a longer wavelength of sound.

There is a very similar Doppler Effect for light. When something is moving towards you, its light is shifted to shorter wavelengths, which we detect as bluer light; we call this a “blueshift”. When something is moving away from you, its light is shifted to longer wavelengths, which we detect as redder light; we call this a “redshift”. Unless we’re looking face-down on the orbits of planets around another star, the gravitational effect of those planets on their parent star as they go around it will cause successive blueshifts and redshifts in the star’s light as the planets go around the star.

If you think about it, the easiest planets to find using this method should be large planets– planets the size of Jupiter, or perhaps even larger– very close to their star. That’s because larger planets will have a greater gravitational pull on their star. Furthermore, planets will also have a greater gravitational pull on their star if they are closer to that star. Finally, planets close to their star will have shorter orbits than farther planets, so you wouldn’t have to wait as long to see a change in the redshift or blueshift of the the light from the star as the result of a planet going around the star. And, indeed, most of the planets found using this “wobble” technique for quiet some time were what astonomers dubbed “hot Jupiters”– gas giant planets extremely close to their parent star. However, as time went on, more and more data built up, and planet-hunters started finding stellar systems that better resembled our own Solar System.

Gliese 581 is an interesting system, in that even before the latest discoveries it had four known planets. Indeed, a couple of them are “super-Earth” sized– perhaps six times the mass of the Earth– and were tantalizingly close to the habitable zone of their star. The habitable zone is the range of distances from the star where you can find liquid water. If you’re too far away, it will all be snow or ice. If you’re too close, it will all evaporate. But if you’re just right– hence the reference to Goldilocks– then you can have liquid water. Life as we know it on Earth depends on being in an environment where you can have water in the liquid state.

On September 29 of this year, astronomers Steven Vogt, Paul Butler, Eugenio Rivera, Nader Haghighipour, Greg Henry, and Michael Williamson uploaded a paper to the arXiv.org preprint server annoucing the discvoery of two new planets orbiting Gliese 581. This paper has been accepted for publication in a peer reviewed journal, The Astrophysical Journal. In this paper, they announce the discovery of two new planets orbiting Gliese 581, including Gliese 581g, the planet you’ve heard so much about. This planet is only 15% the distance from its star that the Earth is from the Sun, but the star is a red dwarf star, enough dimmer than the Sun that this puts the planet smack in the middle of the habitable zone.

You have to think about the data that led to this discovery, however. We’re talking about a star that has six planets oribitng it. Because the effect of even one planet’s gravity on its star is very slight, it’s a tremendous observational feat to tease the redshifts and blueshifts out of spectroscopic measurements of the star due to the star’s motion as a result of that one planet. But that’s a relatively simple motion, a direct back-and-forth as a result of our looking at a projected circle. Once you start adding more planets, at different distances from the stars and thus orbiting with different periods, the motion of the star gets more and more complicated, with wobbles on top of wobbles. It takes serious data processing to disentangle it all and figure out what planets there are to give rise to the complicated sequence of redshifts and blueshifts that you see.

Vogt and his collaborators caution in their paper that it’s possible that the modelling they did of their data to determine the existence of this planet doesn’t provide a unique solution to the problem of fitting a model to the observed sequence of redshfits and blueshifts. It may be, especially if you expand your search to less likely types of orbits, that there a different set of putative planets could give rise to the same observations. They don’t believe this is likely, but admit it’s possible. What’s more, they caution that because this is a hard enough technique, it’s possible that there may be small systematic instrumental effects in their data that could be creating a false signal.

Because this is a very slight signal, in a very complicated system, astronomers are cautious and consider it an unconfirmed planet, despite the media hype about how we have discovered a planet that, to some reports, can definitely support life. Indeed, a week or so ago, Francesco Pepe, announced that he was not able to see the effect of this planet in the data taken with one spectrometer over the course of 6.5 years. Does this mean the planet doesn’t exist after all? No; these are weak signals, after all, and it may be that you need all the data that Vogt and collaborators used to even tease out the signal at all. However, it does underscore that we should temper our enthusiasm while more data is obtained and the existence of this planet is further tested.

This doesn’t in any way diminsh the excitement of finding the first really strong candidate for a planet that’s in the habitable zone around its star. Indeed, given that we’re finding more and more planets all the time, and that we’re getting better and better at finding Earth-sized planets, it’s only a matter of time before we know of quite a number of planets the right distance from the star.

And then? Who knows! Keep watching. That’s the great thing about science; whatever comes, you can be sure that something exciting is waiting just around the corner.

End of podcast:

365 Days of Astronomy
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