Exoplanets: How the Milky Way is Surprising Scientists

EARLIER THIS YEAR, astronomers announced that, beyond our solar system, there are hundreds of possible planets in a small region of the Milky Way Galaxy. These potential planets range from gaseous planets much larger than Jupiter to suspected rocky planets a few times more massive than Earth. As of September 13, researchers had confirmed 20 of these 1,235 candidates are actual planets.

• JACK J. LISSAUER, a space scientist in the Space Science and Astrobiology Division at NASA’s Ames Research Center in Northern California, and a co-investigator on the Kepler space telescope mission;
• GEOFFREY W. MARCY, a professor of astronomy at the University of California, Director of U.C. Berkeley's Center for Integrative Planetary Science, and a co-investigator on the Kepler space telescope mission; and
• SARA SEAGER, a professor of Physics and the Ellen Swallow Richards Professor of Planetary Science at the Massachusetts Institute of Technology. Seager also is a faculty member at the MIT Kavli Institute for Astrophysics and Space Research (MKI).

The following is an edited transcript of the teleconference on the symposium.

THE KAVLI FOUNDATION (TKF): Earlier this year, NASA announced that Kepler had discovered hundreds more planet candidates in the small region of the Milky Way it is observing. That brings the total number of exoplanet candidates that Kepler has identified to date to 1,235. Ongoing studies, meanwhile, are revealing a wide variety of planetary arrangements. How is Kepler changing the way you think about the diversity of extrasolar systems?

JACK J. LISSAUER: In a good fraction of extrasolar systems, we are seeing planets clustered close in. This is something that Geoff was finding with giant planets, Jupiter-mass planets, beginning a decade ago. But we’re now seeing this for smaller planets as well. We’re also seeing that a good fraction of planetary systems tend to be fairly flat like our own solar system. And, there are some systems, like Kepler 11, where there are pretty big planets spaced quite close to each other.

GEOFFREY W. MARCY: The diversity of planetary systems is extraordinary. We had observed unexpected types of planets prior to Kepler, most surprisingly Jupiter-sized planets orbiting very close to their host stars and numerous planets that were in non-circular orbits (eccentric, elongated orbits). In some systems the large planets orbit near the star and the small planets orbit farther out, unlike our solar system that has this neat architecture of small planets in circular orbits closer in and the large planets farther out. So the backdrop for Kepler consisted of the unexpected from previous surveys.

The most dramatic result from Kepler so far comes in two forms: one is the discovery that there are more small planets than large planets in our Milky Way Galaxy. There are almost certainly way more of the smaller planets, almost down to the size of the Earth, than planets the size of Neptune, Saturn or Jupiter. That’s an extraordinary result. It doesn’t speak to diversity, but it certainly carries the profound implication that our Milky Way galaxy has more of the smaller planets that are mini-Neptunes and nearly Earth-size.

The other remarkable discovery is that approximately 120 Kepler stars have been found with two or more planets that transit the star, which indicates that in many cases planetary systems have planets that reside in the same flat plane. This flat structure is exactly what we see in our own solar system. That’s a sort of anti-diversity, if you will, in the sense that flattened planetary systems were expected both from our Solar system and from the flat protoplanetary disks within which planets form. We have learned that planets must have all formed in some kind of flattened disc of gas and dust around young stars.

"There are almost certainly way more of the smaller planets [in the Milky Way Galaxy], almost down to the size of the Earth, than planets the size of Neptune, Saturn or Jupiter. That’s an extraordinary result." — Geoffrey W. Marcy

SARA SEAGER: In my mind, the most significant insight from Kepler so far is that smaller planets are more common than their larger counterparts. One of my favorites is Kepler 10b, which is really quite an extreme planet because it is so very close to its host star, with a less than one-day period orbit. We additionally call Kepler 10b a super-Earth, a planet that is more massive than Earth but is still expected to be predominantly rocky. We think the planet is heated to well over 2,500 degrees Kelvin, and this means that the planet should have liquid lava – not created by volcanoes but just created from the star heating that surface to very hot temperatures, hot enough to melt rock.

TKF: When we think of the habitable zone in a planetary system – the orbital distance from a star where liquid water on a planet could exist – we only have one example to go on, our own solar system. How are the results from Kepler, as well as other exoplanet studies, broadening our perspective?

SEAGER: The diversity of exoplanets has really forced us to reconsider what the habitable zone really is. For example, some of these super-Earths are massive enough that they could retain a different atmosphere than we have on Earth. These super-Earths may hold on to the light gases, hydrogen or hydrogen and helium. In this case, if they have a massive atmosphere they could have a massive greenhouse effect. This could actually increase the range of the habitable zone in a planetary system.

MARCY: We really can’t say anything about the frequency of Earth-sized planets, of rocky planets, that reside in the habitable zones of solar-type stars. We need to collect more data, and we’re about a year away from really saying anything clear and useful on that front.

But I strongly agree with what Sara said. We have very Earth-centric views of what conditions are necessary for life, and here we happen to reside on a planet in a zone that we’ve deemed, we humans have deemed, the habitable zone. But we now know there are many other types of planets, maybe even moons around planets, where there could be the conditions necessary for life. So we’re beginning to broaden our perspective about what types of planets and environments might be suitable for life. The traditional habitable zone might be too narrow.

LISSAUER: I agree with what Sara and Geoff said, but let me just add a few things. People who introduced the concept of the habitable zone defined it as the region at a certain distance from a star where a planet that is like Earth, with a similar greenhouse effect, would have liquid water on its surface. That’s the defining criteria for habitability for life on Earth. However, Geoff has discovered many planets that are in this habitable zone that are not viewed as potentially habitable worlds because they’re way too big.

Meanwhile, there can also be planets that are not in this definition of the habitable zone which could be potentially habitable worlds – for the reason that Sara said. They could have a much bigger greenhouse atmosphere.

TKF: In our own solar system, Jupiter’s moon Europa is believed to have liquid water beneath its icy surface. Scientists suspect this partly because tidal forces exerted on Europa as it orbits Jupiter may generate enough heat beneath the ice to support life. Europa, of course, is far beyond the defined habitable zone in our own solar system. Should we therefore be looking for biosignatures on exoplanets or their moons that lie far from their stars?

SEAGER: Let me jump in here. We have to be careful about what is accessible to us remotely. And that is what is in the atmosphere, and what is on the surface of a planet. We do tend to discard (moons), not because they’re not important but because they’re not exoplanets and we don’t really have a way to study them. I just want to throw something else out here. People do like to talk about life in liquids other than water, and people like to talk about (Saturn’s moon) Titan. Titan has lakes of liquid, not water but liquid ethane and methane. If there are planets that have those characteristics but are as far away from their star as Titan is from the sun, detecting those chemical signatures will be very difficult because the exoplanet’s reflected or infrared brightness will be low. So we also discount exoplanet versions of Titan, not because we don’t believe life exists there but because, for the foreseeable future, we don’t have remote sensing access to them. We don’t have access to the exoplanet versions of Titan.

LISSAUER: And even if we found something like this, we’re not going to get much information about it. So, the more that a planet seems like Earth, in terms of size and composition (mostly rocky), in terms of the amount of radiation it gets from its star, in terms of the star being not that dissimilar to the sun – those are suggestive that it is more likely to be habitable. But again, this is all an extension of one example.

SEAGER: I do really like the theme of where this conversation is going. The point is that we’re rather limited in the planets that we can study in the future, to look for biosignatures, or signs of life, in their atmospheres and on their surfaces. It’s true that Kepler will find lots and lots of objects, but the Kepler exoplanets are not accessible for follow-up observations because they are too far away and we don’t get enough photons (reflecting off them) to analyze. But we are trying to expand the idea of diversity, so that we have a bigger chance of identifying a habitable world in the future.

TKF: Kepler results so far suggest that in our galaxy, there are many more smaller planets than bigger planets. How do we know this?

MARCY: There’s some fairly simple analysis that one has to do. You can’t just take the planets that Kepler is finding at face value and count them up, and reason is of course that smaller planets are more difficult to find than the bigger planets. Kepler has an easier time finding the bigger planets than the smaller planets. So you take that into account.

And then there’s another factor which very few people know about, but if you think about it it’s obvious. Kepler, because it finds planets by the dimming of the star as the planet crosses in front, is limited to finding only those planets that orbit in a flattened plane that is seen edge on as viewed from the Earth. Kepler misses all of the planets that reside in planes that are tilted – in orbital planes tilted 20 degrees, 40 degrees, 80 degrees – to our line of sight. And so we have to correct for that.

After applying those corrections for those known effects, we can determine what the actual occurrence rate is of planets of different sizes. The bottom line is that small planets predominate in our galaxy, compared with the bigger planets.

TKF: What significance can be attached to that?

MARCY: I’ll give the cup half-full and the cup half-empty answer. If you want to be simple minded, the cup half-full answer would say, ‘Well, these early Kepler results show that the planets nearly the size of the Earth – let me be specific, the size is about twice that the diameter of the Earth – are very, very common.’ And so you might conclude that if planets just a little bit larger than the Earth are common, it therefore must be true that planets having the same size as the Earth are also common.

"This is the beginning of: 'What’s out there? What are the planets known?' And in the future, we hope that our descendants will find signs of life." —Sara Seager

But that’s where the cup might also be half empty, in the sense that we have not yet really been able to measure the occurrence rate of truly Earth-sized planets – never mind Earth-sized habitable planets.

It remains possible that planets that are rocky and the size of Earth are still rare. There are theoretical reasons to wonder whether or not planets devoid of the most common elements of the universe, hydrogen and helium – the Earth being one such planet – are really common or not. How do you form planets the size of the Earth that are missing 99 percent of the most common elements, namely hydrogen and helium? So, it remains possible that planets that have just a little, thin veneer of water and are otherwise rocky may still be rare. And I think that’s the glory of Kepler. We’re still going to answer that question. It’s an outstanding question, and we’ve yet to address it.

TKF: Kepler is looking at a tiny region of the Milky Way, and a very small fraction of stars in the galaxy. How can we make conclusions about the population of planets that could be out there everywhere in the galaxy?

LISSAUER: The region that we’re looking at primarily has stars that are of comparable distance to the center of the galaxy as our own sun. And there are a fair number of stars in that category. Now, we are missing, to a large extent, the very smallest stars, M-dwarf stars, because they tend to be so faint. But we’re getting a statistically good set of data on stars that are comparable to the sun. We’re a statistical mission. The longer we go, the better statistics we’ll have, and looking at 156,000 stars gives us a pretty good sample.

MARCY: Basically the stars we are sampling with Kepler are very similar to the predominant types of stars in the Milky Way Galaxy. So in that sense, while we’re observing stars within a narrow pencil beam in the sky, it’s probably a representative fraction.

TKF: As you zero in on individual planets and analyze their atmospheres for chemical signatures of life, how will your knowledge of Earth’s atmosphere help inform your search?

SEAGER: In general, astronomers and planetary scientists have a good handle on what should be in an exoplanet atmosphere that is in chemical equilibrium. What we see on Earth, our only example of a planet with life, is that our atmosphere is heavily modified by life. As a result, Earth’s atmosphere is out of chemical equilibrium. In our atmosphere, oxygen makes up 20 percent of our atmosphere by volume. If it weren’t for life – plants and photosynthetic bacteria – oxygen shouldn’t be in our atmosphere in any significant amount. We should only have negligible amounts. Ozone, meanwhile, is a photochemical byproduct from oxygen, and therefore it’s also a potential chemical signature of life.

So, in general, we are looking for gases that do not belong in an exoplanet atmosphere. More specifically, people would say we’re looking for an atmosphere that is out of chemical equilibrium.

Now, in a very terracentric way, we can look for oxygen or ozone or nitrous oxide, because those are Earth’s prime biosignature gases. But we also don’t know what nature will give us. Whenever we observe in the future, we should design our instruments to be able to detect a broad range of gases, so we will be flexible to see whatever is out there.

TKF: What’s next on the agenda for exoplanet researchers, post-Kepler?

SEAGER: Kepler is taking us in a whole new direction with exoplanets by giving us statistics. Kepler is going to give us lots of numbers to find out which types of exoplanets and exoplanetary systems are more common.

It’s my goal that eventually we’ll return from statistical studies to individual exoplanets orbiting the very nearest stars. At MIT and together with Draper Labs, we are building a prototype nanosatellite that will be launched in 2013. It’s going to be very complementary to Kepler. Instead of looking at 156,000 distant stars, we hope to survey the very nearest sun-like stars for transiting Earth-size planets. So the return to studying individual exoplanets will be for those orbiting stars that are close enough for detailed follow-up.

This artist’s concept illustrates the two Saturn-sized planets discovered by NASA’s Kepler mission. The star system is oriented edge-on, as seen by Kepler, such that both planets cross in front, or transit, their star, named Kepler-9. This is the first star system found to have multiple transiting planets. (Credit: NASA/Ames/JPL-Caltech)

LISSAUER: Kepler still has a long way to go. The spacecraft’s ability to see multi-planet systems farther from their stars, and smaller planets within those systems, will increase greatly as it returns more data over the next six to ten years. Kepler is performing extremely well, and I am interested in analyzing those data for a long time to come.

MARCY: There are stars in the night sky that are three or four light years away – a sort of cosmic stone’s throw right out our back porch. We should be examining those stars for Earth-like planets. Kepler can’t do it. We don’t have any equipment right now that can do it.

What we need is a new space-borne telescope that can detect the Earth-sized planets that may well be orbiting the nearest stars out our back porch, and take spectra of those planets so that we can analyze them for biosignatures of life.

If indeed small planets are common, as the Kepler results so far suggest, let’s go find them right in our cosmic, or galactic, backyard. And so there’s an appeal that I’m essentially making here that more funding be given to NASA and ESA - and frankly I hope they’re doing this in Japan, China and Canada – for a space-borne telescope that can hunt for Earth-like planets around stars that are so close that someday we can send spacecraft there ourselves to get pictures of those Earth-like planets, up close and personal.

"The spacecraft’s ability to see multi-planet systems farther from their stars, and smaller planets within those systems, will increase greatly as it returns more data over the next six to ten years. Kepler is performing extremely well, and I am interested in analyzing those data for a long time to come" — Jack J. Lissauer

LISSAUER: The goal of major missions such as Kepler is to determine the abundance of Earth-like planets near Earth, and whether they’re more common around solar-type stars than, say, smaller stars. This will determine the best type of star to focus our search on, and how far away we'll need to look to find a few such stars with Earth-like planets. Three or four years from now, this information, combined with the technology of the day, will determine whether we are ready to build an advanced space observatory to go out and look for them, and the best way to accomplish that goal.

TKF: What role do you think the James Webb Space Telescope, as well as the next-generation ground-based telescopes such as the Thirty-Meter and Giant Magellan telescopes, will have in future exoplanet research?

SEAGER: What we hope to do with the James Webb Space Telescope is to repeat what we’ve already done for transiting giant planet atmospheres. We want to be able to look at some select, favorable, super-Earths orbiting in the habitable zones of low-mass M stars. We want to look at the composition of the super-Earth atmospheres and to see perhaps if there are any biosignature gases. That’s the ultimate goal with the James Webb. But the planets have to be transiting, and they have to be transiting small stars.

TKF: And what about the next-generation ground-based telescopes?

MARCY: There’s a lot of momentum now toward building 30-meter diameter ground-based telescopes. My own personal opinion is that the capability of those telescopes toward our understanding of Earth-like planets is limited. Beneath the Earth’s atmosphere, the various techniques we know of for detecting Earth-like planets and analyzing their atmospheres spectroscopically is limited. The Earth’s atmosphere is deadly in all the different ways that you might imagine.

So the future of exoplanet research lies with space-borne telescopes. And I just want to re-emphasize what Sara Seager said about the nanosats. She’s really pointing to the future. We need inexpensive ways of getting great equipment above the atmosphere.

TKF: We are in an incredible era of discovery, aren’t we?

SEAGER: If you want to look back to what we remember from hundreds of years ago, inevitably it is the great explorers. Christopher Columbus didn’t know what he was going to find, and he came across North America. Many of us working in the field of exoplanets believe that thousands of years from now, when people look back at our generation in the early 21st century, they will remember the discovery of other Earths as one of our most significant accomplishments.

This is the beginning of: “What’s out there?” “What are the planets known?” And in the future, we hope that our descendants will find signs of life. We are optimistic that our descendants are going to know that there are lots and lots of Earths with lots and lots of signs of life, and we hope they’re going to find a way to travel to the very nearest planets around other stars. And they’ll look back and they’ll see that we were the ones who started it all.