EPOXI sticks like glue to extrasolar planets

By Phil Plait | February 10, 2008 11:00 am

Weirdly, that title is not as big a stretch as you might think.

NASA’s Deep Impact space probe slammed a large copper block into a comet back in July, 2005. However, the probe sailed on, and astronomers hate to waste a good thing. So the mission was retooled, and split into two major projects: for the Deep Impact Extended Investigation (DIXI) it will continue on to observe another comet called Hartley 2, and for the EPOCh component (Extrasolar Planet Observation and Characterization) it will observe planets around other stars.

The combined mission is now called EPOXI.

The comet rendezvous isn’t until 2010. Until then, EPOXI will be used to carefully observe some stars known to have massive planets. These extrasolar planets (or exoplanets for short) were discovered using ground-based telescopes. Hundreds of planets orbiting other sunlike stars have been found since the first was discovered in 1995, but some are special: as seen from Earth, they pass directly between us and their parent star. These are called transiting planets, and some 33 are known at the moment.

Solar systems are like DVDs: wide but flat. If we see them edge-on, then a planet will pass directly in front of the star every orbit. That’s a transit.

A transit makes a fantastic resource for learning about other planets. Using some relatively simple math, you can get so much information from observing transits: the physical sizes of both the planet and its star, the planet’s density, and even (potentially) whether it has moons and the composition of its atmosphere. And all this can be done without ever having directly seen the planet! The way the light from the star dips as the planet passes in front of it tells us all this information, and more.

EPOXI is in a prime location to do this kind of observation. For one, it’s in space, so Earth’s atmosphere doesn’t muck up the view. Second, it really has nothing better to do then sit and stare at a star for weeks at a time, and that is a huge advantage*. The more transits you see, the better you can analyze them. Also, if there is one transiting planet, there are likely to be more. Remember the DVD analogy above? Seen edge-on, the odds are good that other planets in the system will transit their star, but smaller planets make smaller dips in the starlight, making them difficult to see from Earth. EPOXI may be able to detect lower-amplitude transits like that. It will observe five different stars known to have transiting planets, looking for smaller planets, moons around the bigger ones, and other cool things.

I love this idea. Exoplanets are a fascinating subject, and they make up a field of science that has literally only been around a few years. Some transits can be observed using a backyard telescope! But the view from space is a lot better… and launch costs are pricey. Reusing an existing mission to do new science is an excellent use of resources, and a fantastic way to breathe new life into an older project.

*And the reason I titled this post the way I did.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science
MORE ABOUT: Hartley 2

Comments (12)

  1. YinYang0564

    Your post did not specify (at leat as far as I was able to discern), have there been any observed stars with more than one planet?

  2. Kullat Nunu

    It happens that EPOXI has a serious flaw in its main camera: it is defocused and cannot be corrected (that’s why we didn’t get high resolution images of Tempel 1 nucleus except from the impactor probe). But for extrasolar planet studies involving light curves it is actually very good thing; it is much easier to calculate the brightness of the star if the starlight covers more pixels in the CCD camera.

    Your post did not specify (at leat as far as I was able to discern), have there been any observed stars with more than one planet?

    No. The probability of having second planet transiting is not very high because of the larger orbital distance and differences in orbital inclination. The number of known transiting planets is rapidly increasing, so a double transiting system should be found sooner or later.

    There are no multiple planet systems with one transiting planets, either. However, minor discrepancies in transit times may reveal small planets–even planets similar to the Earth.

  3. Kullat Nunu

    Your post did not specify (at leat as far as I was able to discern), have there been any observed stars with more than one planet?

    Of course, there are several multiple-planet systems with non-transiting planets. Rho1 Cancri has as many as five known planets. Mu Arae has four, a few systems consist of three planets and there are many two-planet systems.

    It is likely that many systems consist of multiple planets, but the outer ones have not been detected yet. The radial velocity method which have been used in most discoveries requires that a planet must complete most of its orbit so that its orbit and mass can be measured. In the case of Jupiter analogs, that may take a decade or more.

  4. RayCeeYa

    Your post did not specify (at leat as far as I was able to discern), have there been any observed stars with more than one planet?

    I just wanted to point out that the first confirmed extrasolar planetary system ever discovered had not just one but three planets orbiting a pulsar. So yes multi-planet systems have indeed been discovered. In fact they are pretty common. If we ever found a system that had only one planet, now that would be unusual.

    I just can’t picture a protoplanetary disc coalescing into one huge planet. It doesn’t make sense to me. On the other hand, systems with multiple planets would seem to be the rule.

    But Kullat is right only time will tell what is an “average” planetary system and what is an unusual system.

  5. Just out of curiosity, is the comet Hartley 2 named after the famous Chicago psychiatrist, Bob Hartley?

  6. Does the transit method only work for planets close to the star? What is the most distant known transiting planet?

  7. andy

    Remember the DVD analogy above? Seen edge-on, the odds are good that other planets in the system will transit their star

    Playing Devil’s advocate here, at present this is uncertain due to the lack of orbital inclination measurements for most extrasolar planets. (Plus the fact that currently all known transiting planets are the only known planets in their solar systems). So far there is only one pair of planets in an extrasolar system for which mutual inclination is known (two of the terrestrial-mass planets orbiting a pulsar).

    While it seems likely that planets do form in a disc, who knows what their subsequent evolution would do to the initial system… if planet-planet scattering is responsible for the eccentric configurations of some systems, there could potentially exist systems of planets which have high mutual inclinations and depart strongly from Ye Olde Sol Systeme Modele of planets orbiting nicely in the same plane.

  8. Max Fagin

    Is EPOXI far enough away from Earth that the transit would occur at a noticeably different time?

    I ask mainly out of curiosity, I can’t imagine what new information such a fact might provide, but it would be a nice confirmation of . . . I don’t know. But it would be really cool!

  9. Kevin Conod

    Not unless he’s discovered it. Comets are named after their discoverer, in this case Malcolm Hartley.

  10. Kullat Nunu

    Is EPOXI far enough away from Earth that the transit would occur at a noticeably different time?

    No. If the probe were a few light hours closer (i.e. at Pluto’s for example) the star than Earth is, it would see the exactly same transits, but a little earlier. It would have to be very far away in order to see transit times changing due to geometry.

    Does the transit method only work for planets close to the star? What is the most distant known transiting planet?

    In theory, it does not matter how distant the planet is. However, in reality the probability of a transit drops when the planet’s orbital distance increases. In addition, more than one transit is needed to make sure that there is a potential transiting planet there (finally, radial velocity measurements are needed to confirm the object has a planetary mass). For this reason all transiting planets are hot Jupiters (with the notable exception of Gliese 436 b, which is a hot Neptune orbiting a red dwarf).

    The most distant known transiting planet is HD 17156 b, which orbits its star in highly elliptical orbit every 21 days. Its transits were found with the help of amateurs. The planet itself was discovered earlier. The COROT team may have spotted more distant planets, but if they have, they haven’t told it us yet.

  11. Kullat Nunu

    OT: How you guys make those quotes? And this blog really needs a preview option!

  12. Kevin Conod: You’ve never heard of the Chicago Psychiatrist Bob Hartley?

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