Interview on Extrasolar Planet

This is so last year, but I want to post this for the interest of general education. The reason I am posting it this late is because I forgot, but now that I remembered, here it is. The reason I am posting this is that in astronomy, the search for extrasolar planet is more relevant than ever. Better and better technologies like the Kepler space telescope are being used to probe the vast expanses of our galaxy in search of habitable planets. The e-mail interview below is one I did for my English research report for college, but I believe you may find it of benefit too. The topic is on the method of searching extrasolar planets and some of the discoveries astronomers have made. The one being interviewed is Christine Pulliam, public affairs specialist from the Harvard-Smithsonian Center for Astrophysics, to whom I am very thankful for spending some of her probably precious time answering my request and allowing me to post this. I hope you enjoy it:Let me start by mentioning that there are two methods used to find most known extrasolar planets: radial velocity and transits (a.k.a. “wobble” and “mini-eclipse”).

In radial velocity, astronomers measure the velocity of the star along our line of sight, which introduces a Doppler shift. If a planet is orbiting that star, then just as the star’s gravity tugs on the planet, the planet’s gravity tugs on the star, making it wobble back and forth. The wobble is small, only about 10 meters/second for a Jupiter-sized planet (compared to a star’s typical orbital speed around the center of the Milky Way of about 200 kilometers per second).

In transit searches, astronomers look for a star that dims slightly as an orbiting planet passes directly between the star and us from our point of view. This, of course, requires very exact alignment. Only a fraction of all stars with planets will line up with us in this way. Also, there are a lot of “red herrings,” such as binary stars where the edge of one star just barely grazes the other, mimicking a planet’s signal. (Radial velocity follow-up can sort this out since a star tugs on another star much more strongly than a planet.)

1.      From history, I read that it wasn’t until the late 90s when planet hunting really took off. Why are extrasolar planets so hard to find?

There are three basic reasons extrasolar planets are hard to find: they’re small, faint, and faraway.

Even a Jupiter-sized gas giant is much smaller and less massive than the star it orbits. So as I noted above, a radial velocity search must make measurements accurate to a few meters/second to find a Jupiter, and must maintain that precision throughout several orbits (which for a Jupiter-like orbit would mean monitoring the star for at least 20 years). A Jupiter-sized transiting planet would dim a sunlike star by 1 part in 100 (one percent).

If you tried to detect the planet directly (i.e. photograph it), you run into more problems. The planet is much fainter than its star. In visible light, the star is a billion times brighter. In infrared light, the star is still a million times brighter. And because stars are so far away, the planet will appear very, very close to this overwhelmingly bright light source. It’s like standing in Boston and trying to see a firefly next to a searchlight in Los Angeles.

2.  Related to the difficulty, how precise must instruments be to find an Earth sized planet?

An Earth-sized planet is extremely difficult to find, which is why we haven’t found one yet (at least around a sunlike star).

The radial velocity signal would be around 1 cm/sec (a wobble of 2 feet per minute). The roiling surface of the star itself can mask a signal that small. An Earth-sized world transiting a sunlike star would dim that star by 1 part in 10,000 (a hundredth of a percent). Only now, with the Kepler spacecraft, have we reached this level of precision.

3.    How can astronomers find specifics about each planet, like mass and orbit? And are there other specifics about a planet that can be found besides the ones I mentioned?

Note: I swapped the order of the next two questions since this explanation feeds into the next.

Radial velocity measurements tell you two things: the planet’s mass and orbit. The size of the orbit comes from the period – how long it takes for the planet to complete one orbit. (Apply Kepler’s law.) The mass comes from the strength of the signal (wobble). Note: the signal strength also depends on the inclination of the system. We get the strongest signal if view the orbit edge-on. Any tilt dilutes the signal and makes the planet harder to detect. If its orbit were face-on from our point of view, we wouldn’t see any radial velocity signal at all.

Transits are valued because they give us more information about the planet. Since we must see the planet’s orbit edge-on to get the transit, we know we’re measuring its total mass (no dilution due to tilt). Also, from the amount of dimming we learn how big the planet is (physical size – relative to the star, so we have to know our stellar physics to get an absolute size).

From the mass and physical size, we can calculate an average density, which hints at what the planet is made of – its composition. Low density = gas giant, high density = rocky.

In a few cases, we have measured the constituents of a gas giant’s atmosphere by measuring the star’s light filtered through the planet’s atmosphere during a transit. For example:

We have also begun to make crude “weather maps” of a handful of gas giants using clever techniques and the Spitzer spacecraft:

4.    What kind of limitations must astronomers deal with when collecting information about planets?

As noted above, the angle at which we view a planetary system – its inclination – restricts what we can learn about a planet’s mass, or whether we can detect a planet in the first place. We’re also limited by the accuracy of our stellar models, since we measure the planet’s mass and physical size relative to its star. Both radial velocity and transit searches require very precise measurements, and the required precision gets tougher and tougher as you go to smaller and smaller planets.

5.    What is, in your opinion, the best method for finding exoplanets?

Transit searches are really coming into their heyday now. If you sort through thousands of stars at a time, you’re bound to find a few with planets that line up just right. And we get much more information from these systems once we find them, since we can learn the planet’s physical size and even detect its atmosphere in some cases. I believe the transit method is the best and will continue to be so for a decade to come.

6.     I know there is a method of finding planets by using gravitational lensing. How viable is this technique? So far, very few planets have been found with it.

The microlensing technique has the advantage that it can find planets far from their stars, in an orbit that would take decades of radial velocity observations to confirm. However, it requires a very exact alignment of the star-planet system with a more distant star, which just doesn’t happen often. It’s much more rare than transiting systems, so you have to monitor even more stars. I don’t see it becoming a particularly productive method of finding planets.

7.    A lot of the planetary systems found so far seem pretty exotic (planets with very elliptical orbits, hot Jupiters, etc) compared to our system. Are there some you can think of that is similar to our solar system?

Yes. The big one in the news recently was the red dwarf Gliese 581, which one group announced to have a “super-Earth” in an orbit that might warm it enough for liquid water to exist. (There’s some controversy over whether that planet actually exists though.)

55 Cancri is a binary star approximately 41 light-years away from Earth. The system consists of a yellow star and a red dwarf star separated by over 100 billion miles. Five planets have been confirmed to be orbiting the yellow primary, 55 Cancri A. Upsilon Andromedae is a similar binary-star system with three planets.

61 Virginis is a sunlike star with three planets, all with masses between 5 and 25 times that of Earth. The three planets all orbit very near the star – when compared to the orbits of the planets in our solar system, all three would reside inside the orbit of Venus.

Probably the best example known right now is HD 10180, a sunlike star located 127 light-years away. It has at least five and possibly seven planets (which would make it the system with the most known planets to date).

Our techniques are better at finding big planets in close orbits, which is one reason we’ve found so many “hot Jupiters.” This is called a selection effect. I expect we’ll find more planetary systems similar to ours as our search methods improve and we have more years of data.

8.    Do astronomers count pulsar planets as ‘planets’? Because I have read written pieces which don’t include them as first discoveries, and NASA’s catalogue directly states they are omitted from the tally.

Ah yes, the poor pulsar planets! I made a point of mentioning them in a public talk I developed on exoplanets last year.

I think they’re less exciting to astronomers because the ultimate goal of planet hunting is to find another planet with life on it. The pulsar planets are bathed in ionizing radiation without any gentle warmth or starlight to nurture life. Dead rocks, even planet-sized ones, aren’t very exciting. They have the advantage, though, of a completely different detection method than any other planet – pulsar timing.

9.    Is there any way to detect signature of life in an exoplanet?

Yes, if we can find a rocky exoplanet with a detectable atmosphere, which would require it to transit its star. Future generations of space telescopes could potentially detect the slight absorption of starlight by specific gases in a distant planet’s atmosphere. We would then look for gases like oxygen and ozone, which come from plant life, or methane, which comes from bacteria. Since oxygen and methane are both reactive, if we found both in significant quantities, this would be a very good sign that life was present and continuously altering the atmosphere (rather than, say, volcanoes or other geological processes).

10.    What are some of the newest, exciting discoveries and development in the field?

The deployment of the Kepler spacecraft, which offers our first real opportunity to find Earth-sized planets. Also the success of ground-based transit searches like MEarth and HAT. The Europeans have the best radial velocity search going right now. Only 15 years after the discovery of the first exo-Jupiter, we’re now on the cusp of finding not one but many Earth-sized worlds.

11. Why is it important for astronomers to find other planets? What do they hope to achieve?

The ultimate goal is to answer the question: Are we alone in the universe? We want to find life out there. It doesn’t have to be intelligent life – any life would be very exciting. And we want to find out how common or rare Earth-like (potentially habitable) planets are. Does our galaxy teem with life, or are we rare and precious?


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