Wednesday, December 8, 2010

Communicating Climate Change

In an effort to combat misinformation about climate change during the UN negotiations in Cancun this week, the American Geophysical Union (AGU) -- a non-profit scientific organization with more than 58,000 members -- launched a Climate Question & Answer Service for journalists. The program is part of a recent effort by scientists to be more proactive in communicating the science of climate change to the public, but it draws a line at questions of policy. In reality the line between science and policy is slightly fuzzy, and scientists need to formulate a coherent strategy to have any chance of success.

Climate scientists have come a long way in their thinking about public relations since the release of the last report by the UN Intergovernmental Panel on Climate Change (IPCC) in 2007. At a press conference in Paris associated with that release, lead author Susan Solomon of the National Oceanic and Atmospheric Administration declined to comment on how society should respond to the climate crisis. "I honestly believe that it would be a much better service for me to keep my personal opinions separate," she said. Her response is now regarded as one of the greatest missed opportunities to frame the public debate about climate policy. Solomon and the IPCC team went on to receive the Nobel Peace Prize for their work, along with Al Gore.

Given the persistent misinformation and outright falsehoods perpetuated by some media outlets and politicians, the new question and answer service is a step in the right direction. Journalists who are unsure about how to report on some technical issue -- or who are confronted with unsubstantiated claims from global warming skeptics -- now have the ability to fact check. As part of the pilot program, more than 700 PhD climate scientists volunteered to answer questions from a shared email box over a period of 10 weeks around the UN negotiations in Cancun. But the "AGU explicitly requests participating scientists not to comment on policy", and questions "relating to policy, ethics, and economics will be returned to sender". In other words "just the facts". Unlike these scientists, politicians and media pundits are not constrained by the facts -- so ultimately this approach may still be a losing strategy.

I have several ideas for a more successful media strategy by scientists. First, the answer to a question about policy does not need to be political to be useful. For example: "Over the next several decades society must dramatically reduce its emissions of heat-trapping pollution into the atmosphere. How this is accomplished, and on what timeline, are questions that must be answered by policy makers." Second, scientists should try to frame climate change as a form of debt being left to future generations. The same conservatives who are concerned about passing a $14 trillion national debt to their grandchildren are also opposed to any action on climate change. Finally, rather than talk about "avoiding the worst impacts of climate change", it's time to focus on the inescapable impacts that we will see in our lifetimes. For example: "No matter what we do now, by the middle of the century the global climate will warm by at least 4 degrees Fahrenheit. The only thing we can control now is the sort of planet that our grandchildren will inherit from us."

By re-framing the debate about climate change policy, and by shifting the focus to the immediate impacts that are both certain and unavoidable, scientists can jump start the necessary response by society. When people understand that it is "too late" to avoid severe impacts during their lifetime, they just might skip over the denial, focus their anger, and begin bargaining.

Wednesday, November 24, 2010

The Cost of Science

With so many recent discussions about reducing the federal budget deficit and gradually paying down the nearly $14 trillion national debt, the government agencies that fund science appear to be easy targets. Looking for solutions to narrow this budget gap, a recent opinion column in my own local newspaper characterized NASA's budget as "a luxury we can't afford". But the numbers tell a different story -- the sum total of all non-defense discretionary spending is less than half the current budget deficit, and funding for science amounts to pocket change buried under a mountain of cash.

The 2010 federal budget totals $3.55 trillion. When you compare this to the $2.38 trillion in revenue from taxes, you get a $1.17 trillion deficit -- the gap between what the government spends, and what it collects from taxpayers. Similar deficits since the 1970's have gradually increased the U.S. government debt, like running a balance on the nation's credit card. The interest payments alone on this national debt will amount to $168 billion in 2010, enough to give every taxpayer in America a $1000 refund. To fix the problem, the government needs to cut spending and/or raise taxes to balance the budget and slowly begin paying down the national debt. It's been done before -- Bill Clinton inherited a $255 billion deficit from George H.W. Bush, and he transformed the federal budget to yield a $236 billion surplus by the time he left office. But this episode of fiscal responsibility was short-lived, and the nation has been spending hundreds of billions of dollars more than it collects ever since.

So, how does science funding compare to this enormous imbalance between taxes and spending? Suppose lawmakers decided to completely eliminate the National Science Foundation -- how much money would it save? The 2010 NSF budget is just $7 billion, about 0.6% of the current federal deficit. By contrast, the Department of Defense spends the annual budget of the NSF every few days. If lawmakers wanted to be more selective in their cuts to science, they might decide that Mathematics and Physical Sciences are "a luxury we can't afford". The MPS budget in 2010 is $1.38 billion, a potential savings of about $8 for every taxpayer. If this sort of cut seems too draconian, maybe congress would just target Astronomical Sciences with an annual budget of $250 million, the equivalent of a round-off error in the federal budget. You get the picture -- there isn't a lot of savings to be realized by forcing the nation's scientists into the unemployment lines.

What about NASA? Surely the potential savings from the space program could make a significant impact on the deficit -- right? At $18.7 billion, the annual budget of NASA is larger than the NSF, but it still represents just 1.6% of the deficit or about 10 days of military spending. NASA launches astronauts into space to repair a 20-year-old telescope that continues to make ground-breaking discoveries on a weekly basis -- it captures the imaginations of children around the world and inspires them to study science. But much of NASA's budget is actually devoted to spaceflight -- the Science Mission Directorate has an annual budget of $4.5 billion, with about $1.1 billion for Astrophysics. It's a good deal of funding compared to the NSF, but it barely registers in the context of total government spending.

Congress has some difficult decisions to make in the coming years. It's clear that as a nation we cannot continue to spend more than we are willing to pay. Returning tax rates to the levels of the 1990's is certainly part of the solution, but spending cuts will also be necessary. Our investments in science can be sustained by trimming the defense budget just a few percent. Let's hope our lawmakers arrive at the right conclusions.

Friday, October 22, 2010

NASA Outsourcing Science

Next week NASA will issue a press release and hold a media event in Denmark to announce the first results from the study of pulsating stars with the Kepler satellite. Kepler was designed to find distant Earth-like planets, but part of the mission is devoted to characterizing the target stars. When NASA holds a press conference, the media usually report on whatever they say -- so this is a great opportunity for a group of scientists who have been working in relative obscurity. Unfortunately, the public may never hear about the most significant results.

Like many NASA projects, the history of the Kepler mission has been riddled with delays and cost overruns. In 2006 NASA approved a 20% increase in the price tag for Kepler, bringing the total cost of the satellite to $550 million and pushing the launch date from 2007 to 2008. A year later the team asked for another $42 million -- but this time the request came to the desk of Dr. Alan Stern, the new associate administrator for NASA's Science Mission Directorate. "Four times they came for more money, and four times we told them no," said Dr. Stern. With threats of NASA turning the project over to new management, it was widely reported that the team cut costs by reducing the mission duration from 4 to 3.5 years and scaling back on preflight tests of the hardware. It is less well known that they also reduced the budget by outsourcing one of the major scientific programs to Europe, where they could externalize the data analysis costs to international partners.

In addition to the primary mission of discovering habitable Earth-like planets around distant stars, the Kepler satellite also has the capability of studying the stars themselves in great detail. The giant digital camera inside the space telescope monitors the brightness of more than 150,000 stars every 30 minutes. This is enough to detect the tiny drops in brightness caused by planets that pass in front of their host stars -- events that typically last for a fraction of a day. But at any given time, the brightness of 512 of those stars is measured 30 times more often. This 1-minute sampling is enough to document the subtle pulsations of starlight caused by continuous "star-quakes" that reveal the properties of the star itself. Thousands of stars have been monitored for one month each during the first year of the mission, and the entire program has been coordinated from headquarters in Denmark at no cost to NASA. Some of the world's leading experts in the study of pulsating stars are based in Europe, so they traded their labor for early access to Kepler data -- effectively extracting a subsidy to NASA from their home countries.

After the first year of the mission, the initial results from these studies were about to be published, so NASA asked the largely-European team to draft a press release. With the help of one of their local media offices (another hidden subsidy), the team pitched a story about Kepler "taking the pulse of distant stars" to learn about their properties -- while developing techniques that could also be used to characterize the host stars of the many planets discovered by the mission. After receiving the draft press release, NASA delayed the target date for the media event by two weeks and asked for additional information from the lead authors of the studies, essentially trashing the draft and starting the process from scratch. This week NASA circulated their "final" version of the press release to the scientists, which largely ignores many of the results and gets the science wrong in what remains -- leaving the scientists to wonder why they agreed to embargo their results for two months.

Kepler Program Scientist Douglas Hudgins was positively giddy about the scientists "quite literally revolutionizing our understanding of stars ... at no cost to the American taxpayer". He's absolutely right about the quality of the work being done by these world-class researchers -- but because of continued mismanagement at NASA, you might never hear about it. I guess you get what you pay for.

Monday, September 20, 2010

Astronomical Events

One of my astronomy professors had a cartoon on her door labeled "a beginner's guide to star gazing". The first panel showed a person looking downward shielding his eyes with his hand, and the caption said "wrong". The second panel showed the same person looking upward, and the caption said "right". Star gazing isn't rocket science -- anyone can do it. Even so, last month many people were fooled by an Internet hoax claiming that Mars would appear in the night sky as large as the Earth's moon, for one night only! While astronomers might lament the inability of the general public to recognize this claim as a hoax instantly, it may have been credible precisely because of the way astronomers and the media typically report celestial events.

For example, it has been widely reported that Jupiter will be closer tonight than it has been since 1963 or will be again until 2022. This is technically true, but it makes the opportunity of viewing Jupiter tonight seem like an all-or-nothing proposition -- if you miss it tonight, you'll have to wait 12 years before you can see it this good again. The reality is not nearly as urgent. Tomorrow night, Jupiter will be 0.003% further away than it will be tonight. Next week it will be 0.2% further away, and when the Earth swings by Jupiter again next year (28 October 2011), it will be 0.4% further away than it was this year. Even a trained eye would have a hard time detecting differences this small. Despite this reality, public observatories will be jam packed tonight while almost nobody will show up tomorrow.

Maybe we can understand this tendency of the media to hype astronomical events by considering another recent example -- the "International Observe the Moon Night" that took place over the weekend. This event was not fabricated by the media, it was created by astronomers to generate increased public interest in the Moon. For one night, several NASA centers partnered with local amateur astronomers to set up telescopes and open their doors to public viewings of the Moon. Prominent scientists gave lectures about the Moon that were streamed live online. There was nothing urgent happening on the Moon over the weekend. It was just a concerted effort by lunar scientists to engage the public, and it worked. Meanwhile, anyone can observe craters on the Moon at any time from their own back yard with a cheap pair of binoculars.

Maybe the lesson here is that the media understandably tends to focus on what is new or unique, so astronomers pitch celestial events as if they were Valentine's day or Mother's day -- a social construct reminding people to do what they could be doing all of the time (appreciating their partner/mother, or looking at the sky instead of watching reality television).

Monday, August 23, 2010

Forcing the Climate

In an editorial for the Houston Chronicle earlier this month, Walter Cunningham (a geophysicist and former Apollo astronaut) claimed that global warming is a natural phenomenon that is unrelated to human activities. "Scientists have long known that the sun, oceans and variations in the Earth's orbit are the principal drivers of climate change", he wrote. It is actually true that changes in the Earth's orbit can explain the periodic ice ages and warm periods of the past. But all of the known sources of natural variation cannot account for the warming observed in the last few decades.

Three properties of the Earth's orbit are known to change on long timescales, from 26,000 to about 100,000 years. These include how non-circular the orbit is (the eccentricity), how titled the rotation axis is compared to the orbital plane (the obliquity), and the direction of that tilt at a given point in the orbit (precession). The effect of these variations on the length of the seasons and the amount of sunlight received by the Earth throughout the year was worked out in the 1930's by a Serbian mathematician. Comparing the predictions of this natural variation in solar energy received by the Earth with the reconstructed surface temperature over the past 250,000 years shows a clear correlation, at least until recently.

What about the Sun itself? Could variations in its energy output be responsible for recent changes in the Earth's climate? In fact, the magnetic field of the Sun flips every 11 years and during this time the total energy output changes by about 0.1 percent -- the Sun emits more energy during periods of high magnetic activity. However, the change in energy output from one magnetic cycle to the next can only account for about 10 percent of the warming over the past century, and it has actually been going down since the 1960's while the temperature of the planet has continued to rise dramatically.

Climate scientists have tried to account for global warming using only these natural contributions, but they can only match the recent increases in temperature when they also consider human activities. The most significant of these activities is the release of heat-trapping pollution like carbon dioxide and methane, but the scientists also consider the cooling effects of aerosols and modern changes in land use patterns. On balance, considering all of the natural and human-induced contributions to climate change, the conclusion is unambiguous. The planet is warming because of us.

It is human nature to prefer a reassuring lie over an inconvenient truth. We want an easy way to escape the difficult situation we have created for ourselves. In a rebuttal to Cunningham's editorial, Nobel prize winner Robert Curl put it simply. "How much does the present owe to the future? This is a hard philosophical question. Neither Cunningham nor I will live to see how this turns out, but I expect my grandchildren to. I prefer that the planet they inherit is not a world in distress."

Tuesday, July 13, 2010

Cycles in the Sun

In a classic Far Side cartoon by Gary Larson, we see a man sitting at a desk surrounded by fans, reading a newspaper. On the wall beside him is a large lever with two positions labeled "rise" and "set", and the caption reads: "Inside the Sun". Our nearest star is actually much more dynamic than it might first appear, and a short paper published this week has revealed fresh evidence of something very interesting beneath the surface.

To anyone who has ever seen the Sun through a telescope (equipped with a special filter to protect your eyes!), the most obvious features are sunspots -- small dark patches about the size of the Earth that are strongly magnetic and cooler than their surroundings. If you recorded the position of the spots every day for weeks, you would see them come and go tracing the fluid rotation of the Sun -- moving faster near the equator and more slowly towards the poles. If you kept careful records of the sunspots over decades, you would notice a regular rise and fall in the number of spots every 11 years. This is the most visible manifestation of an underlying magnetic cycle in the Sun, where the magnetic bubbles that appear as sunspots are periodically stretched out, reorganized and recycled by the rotation and other motions deeper in the interior.

Although we cannot see spots on other stars directly, long-term studies of stars like the Sun show similar magnetic cycles. All other things being equal, stars that rotate faster generally have shorter cycles, since they are more efficient at recycling their magnetic bubbles. However, when astronomers examine in detail the relationship between rotation and the length of the magnetic cycle, there seem to be two different types of stars -- "active" stars that spin faster than the Sun and have magnetic cycles every 400 rotations, and "inactive" stars that spin more slowly and exhibit magnetic cycles every 90 rotations. Some of the "active" stars show both types of magnetic cycles simultaneously, suggesting that the two types of cycles might actually just be operating in different regions of the star.

The paper published this week by a team of mostly British astronomers used an innovative tool to study the magnetic cycle of the Sun -- they peered beneath the surface with the help of sound waves that bounce around inside and set up standing waves with specific frequencies, a technique known as helio-seismology. As the Sun moves through its magnetic cycle, the frequencies of these standing waves change slightly. Looking at the changes in these frequencies over 25 years the team noticed not only the expected 11-year variation, but also a regular 2-year variation that appeared to be operating independently. Although the Sun itself seems to be peculiar, other stars that show an "active" magnetic cycle every 11 years also show a secondary "inactive" magnetic cycle -- every 2 years. This is the most direct evidence to date that the Sun might actually have two different magnetic cycles operating simultaneously on the inside.

Although the Sun does not seem to be a typical star, astronomers can study it in much more detail to understand how stars work in general. As this new observation suggests, it is equally important to study a variety of other stars to identify what is peculiar --- and what is normal -- about our nearest star.

Friday, June 11, 2010

Exoplanet in Action

Yesterday the European Southern Observatory issued a press release containing an incredible image of an alien planet moving from one side of its host star to the other in just 6 years. The star, known as beta Pictoris, has been studied for several decades after the early discovery that it was surrounded by a debris disk. With new technologies that now enable high-resolution imaging from large ground-based telescopes, we can actually watch an exoplanet as it moves around in orbit.

In 1983, beta Pictoris was one of four hot stars (including also Vega, Fomalhaut, and epsilon Eridani) that were discovered to be surrounded by disks of gas and dust using the IRAS satellite. The classic image of beta Pictoris at the center of a cross-hair with the debris disk shooting out diagonally in 1980's graphics instantly became an icon -- symbolic of the quest for extra-terrestrial life. If we could see an alien solar system being formed before our eyes, then the chances seemed good that we are not alone in the Universe. It was just 18 months ago that NASA released the first direct image of an exoplanet from the Hubble Space Telescope, in orbit around one of the other IRAS stars, Fomalhaut. But in that case the orbit of the planet was 10 times the size of Saturn's orbit around the Sun -- so the planet only moved a tiny fraction of its orbit in the two years between the sets of observations. The planet around beta Pictoris is much closer to its star, at a distance between the orbits of Saturn and Uranus in our solar system.

Soon after the Hubble announcement in 2008, a team of French scientists released their first image of a faint object close to the star beta Pictoris, from observations made at the Very Large Telescope in 2003. The telescope has a computer-controlled mirror that can actively deform itself to compensate for the distortions caused by turbulence in the Earth's atmosphere. Using this technology, the scientists were able to cleanly separate the image of the faint object from the image of the bright star, just one ten-thousandth of a degree apart in the sky (roughly the size of a mosquito viewed from a mile away). The trouble was, they couldn't rule out the possibility that the faint object was a distant background star -- so they had to wait several years to see if the object would move. In late 2009 they obtained a second high-resolution image, and sure enough the faint object had moved to the other side of beta Pictoris. It was a true planet, about 9 times as big as Jupiter.

The astronomers estimate that they should be able to see the planet go all the way around beta Pictoris in less than 20 years. By then this type of observation should be routine, and it would be surprising if we weren't monitoring the movements of hundreds of such exoplanets.

Friday, May 28, 2010

The Ages of Stars

Astronomers have used many tricks to try and measure the ages of stars, but most methods are not very precise. A star like the Sun has a lifetime of around 10 billion years, so knowing the age to within a few billion years is about the best we can expect from conventional techniques. Using new observations from the Kepler space telescope, this week I finished a project that measured the age of a star to within just 50 million years, a precision that has only been achieved for one other star: the Sun.

We can set a lower limit on the age of the Sun just by measuring the age of the Earth. This is usually established by finding rocks containing elements (like Uranium) that radioactively decay, and determining what fraction of the material has been converted into the stable byproducts. The oldest rocks on the Earth are found to have an age around 4.4 billion years, while similar methods applied to meteorites give an age just over 4.5 billion years. This is a good first guess for the age of the Sun, but there are also more direct methods. As a star gets older, it converts hydrogen into helium and energy through nuclear fusion -- this is what makes a star shine. Sound waves generated by the boiling motions of the hot gas near the surface travel through the center of the Sun and reveal the composition deep inside. This technique, known as helio-seismology, implies an age for the Sun of 4.68 billion years.

The Kepler space telescope is observing thousands of other stars like the Sun, and we can also use seismic techniques to measure their ages. For a typical star, astero-seismology can determine the age to within about 1 billion years. However, during certain phases of a star's life we can see an interaction between the sound waves generated at the surface and buoyancy waves (similar to the waves in the ocean) that are trapped deep inside. Essentially the two types of waves couple to one another briefly and influence the tiny changes in brightness that we can measure at the surface. The net effect is like a very precise clock -- by measuring the frequency of the brightness variations, we can tell the age of the star to better than 100 million years. To make such measurements, we rely on computer models of the star -- so any imperfections in our models carry over into uncertainties about the absolute age. It's as if we have a watch that keeps very good time, but we still don't know whether it is set to the correct time zone.

It is difficult to measure the ages of individual stars, since the visible changes throughout their lifetimes are fairly subtle. Whatever the limitations of seismology for determining the absolute ages of stars, this technique can certainly place stars into a precise chronological sequence. By applying the method to many other stars like the Sun we can get a better understanding of what our own star was like in the past, and how it will be in the future.

Tuesday, April 27, 2010

Life in the Universe

Legendary physicist Stephen Hawking made headlines this week when he suggested that "if aliens ever visit us...the outcome would be much as when Christopher Columbus first landed in America, which didn't turn out very well for the Native Americans." Most people are surprised to learn that many astronomers actually believe in extraterrestrial life, though few scientists tend to speculate about how the aliens might relate to our civilization. But what we know about the size and composition of the universe -- and what we've learned about the stars in our own Galaxy over the past 20 years -- make it unthinkable that we could possibly be alone.

The scientific argument for extraterrestrial life can be traced back to the 1960's, when astronomer Frank Drake first formulated what has come to be known as the "Drake Equation". Essentially, this is just a simple method to estimate the number of intelligent civilizations in our Galaxy. It starts with the number of stars (a very big number) and then multiplies by reasonable estimates of the fraction of stars that have planets, the fraction of those that are habitable, that form life, where the life becomes intelligent, that develop the technology to emit signals into space (like radio communication), and finally how long those civilizations exist before becoming extinct or destroying themselves. The main conclusion is that even if you make very pessimistic assumptions for all of these fractions, the number of stars is so large that there will still be a handful of intelligent civilizations like ours in the Galaxy. In Hawking's words, "To my mathematical brain, the numbers alone make thinking about aliens perfectly rational."

It was just 15 years ago that the first planet outside of our solar system was discovered around a star like the Sun. As of this week, more than 450 "exoplanets" have been discovered around other stars. Many of these planets do not resemble those in our own solar system at all. The methods that have been used to discover them are biased towards large planets like Jupiter that orbit relatively close to their suns like Mercury (and in most cases even closer), but a few of the known exoplanet systems include up to five planets. The diversity of planetary systems that astronomers are finding around other stars suggests that planets are much more common than we originally thought, and it's a first step towards actually measuring the fractions in Drake's equation.

Despite our progress in detecting planets around other stars, we still don't know of any habitable planets other than the Earth. However, within the next few years NASA's Kepler mission will tell us exactly how common habitable planets are in our Galaxy. Launched just over a year ago, the Kepler space telescope is performing a detailed census of planets around stars in our Galactic neighborhood. Its enormous digital camera has the sensitivity to detect the tiny eclipse of a planet the size of the Earth passing in front of its host star, and the mission is monitoring more than 150,000 stars like the Sun for such signals. Since a habitable planet like the Earth takes a full year to complete an orbit, Kepler needs to search for several of these "transits" over the course of a few years to identify the distant Earths reliably. If such planets are as common as we believe, Kepler should find a dozen or so within the first few years -- but in any case, it will measure another fraction in Drake's equation.

Throughout human history whenever we have believed ourselves to be special in some way, we turned out to be wrong. We now know that the Earth is just one planet among many in our Galaxy, and we will soon know how common planets are that might also be habitable. With a short list of distant Earths, we can begin to search for alien radio signals. It may take some time, but the conclusion is inevitable -- we are almost certainly not alone.

Tuesday, March 16, 2010

Comet Crash

Last week, astronomers watched in awe as several anonymous comets plunged into the Sun. Many people are surprised to learn that similar events happen almost every day, but the icy fragments are typically too small to be seen. A string of comet fragments struck Jupiter in 1994, causing quite a splash. Should we worry that the Earth might be next?

Comets are generally believed to be leftover debris from the formation of our solar system more than 4 billion years ago. A distant icy halo surrounding the Sun, known as the "Oort cloud", is the source of nearly all of the comets that have been observed throughout history. Some of these comets return regularly (like Halley's comet in its 75.3 year orbit), while others pass through once and head back to the cloud, never to be seen again. Astronomers believe that comets are like "dirty snowballs", containing a loosely packed mixture of ice and dust. As they approach the Sun they warm up and begin to evaporate, spawning a long bright tail that generally makes them much more visible.

There is a population of comets known as the "Sun grazers", which appear to be the leftover fragments of a much larger comet that probably broke up more than 2000 years ago. The orbits of these comets are extremely elongated, and in some cases shoot them directly into the Sun, a fiery demise that is barely noticed by the enormous boiling mass. Most of the fragments are so small that they are nearly invisible until they begin their plunge, where satellites that are always watching the Sun for signs of hazardous space weather can finally see them brighten and abruptly disappear.

In the summer of 1994, the planet Jupiter had a similar encounter with a train of comet fragments known as "Shoemaker-Levy 9". About two years earlier a large comet passed so close to Jupiter that it was broken into many pieces by tidal forces and thrust into an orbit that would strike the giant planet like a string of pearls on the next pass. I was an undergraduate student at the time, working with a group known as SpaceWatch at the University of Arizona. Over the months leading up to the collision, we carefully measured the positions of each fragment to calculate the individual orbits and make predictions of the precise impact times. For the nine largest pieces, we calculated times that were correct to within a few minutes.

As for something similar happening to the Earth -- don't worry! Jupiter is a huge target with significant gravitational pull, and the Sun is over 1000 times more massive. Nothing as large as a comet has struck the Earth since the disappearance of the dinosaurs, 65 million years ago.

Friday, February 26, 2010

Red Giant Groove

Earlier this month I attended a scientific conference on a small Spanish island called Lanzarote, just off the coast of Morocco. The meeting was for scientists who study the insides of the Sun and other stars using a technique similar to seismology. Without a doubt, the coolest result presented at the meeting came from Italian astronomer Andrea Miglio and his colleagues in Belgium. For the first time in any star other than the Sun, they measured an abrupt variation in the internal structure -- like the boundary between the crust and the core of the Earth, but in a red giant star more than 350 light years away.

A red giant star is what our Sun will become when it begins to run out of hydrogen fuel, about 5 billion years in the future. As the hydrogen begins to burn faster in a shell around the helium core, our star will slowly bloat and cool -- eventually engulfing the inner planets, including the Earth. Like the Sun, the surface of a red giant seems to boil as convection brings heat from below and radiates it into the coldness of outer space. The boiling churns more slowly in a red giant, but the turbulent motions still create sound waves that travel down through the star and then back toward the surface. Some of these sounds have just the right pitch, a million times lower than we can hear with the human ear, and they set up standing waves that cause the entire star to slowly change its brightness over hours and days.

Like the standing waves on a vibrating string, the brightness changes that we can see in a red giant have very specific frequencies. The fundamental frequency of a vibrating string is sort of like a jump rope, with the entire length moving up and down together. If the people at the ends of the rope move their hands up and down at twice the fundamental frequency, the standing wave will have two lobes with a stationary point in the middle -- one side will move up while the other moves down. At three times the fundamental frequency you get three lobes, and so on to create a whole series of evenly spaced frequencies. Now imagine that somewhere along the jump rope you attach a small lump of clay -- the extra weight at this position will change the way that waves travel along the rope, and the frequencies of the standing waves will change slightly from the evenly spaced pattern. Here's the cool part -- by looking at the deviations from uniform spacing, you can actually figure out where along the rope you have attached the clay!

This is essentially what Andrea and his friends did for the red giant. Using measurements taken over 5 months from a French space telescope called COROT, they identified a series of almost evenly spaced frequencies of brightness variation in a red giant known as "HR 7349". By looking at how the frequencies they observed differed from perfectly uniform spacing, they determined the position of a boundary layer on the inside of the star. Comparing this prediction with theoretical models of the star, they identified this layer as the depth below the surface where the temperature was just high enough to strip both of the electrons from every helium atom. From a distance of more than 350 light years, the team had pinpointed the position of a subtle change in the composition and density of the star -- just by measuring its regular changes in brightness over time.

This beautiful result promises to be just the first demonstration of the types of measurements that are possible using stellar seismology. The same basic techniques will allow us to measure the size of the boiling layer on the surface of a star like the Sun, providing new ways to test our understanding of how stars are actually built.

Wednesday, January 27, 2010

Kepler's First Planets

This month the Kepler mission announced its first batch of extrasolar planet discoveries. Many people were surprised that only five new planets were announced, since the mission is monitoring more than 150,000 stars. But there are very sensible reasons why the initial catch was limited.

The first thing to understand about the newly discovered planets is that all of them were found in just the first 43 days of Kepler observations. The satellite was launched in early March, but the first two months were spent largely on engineering to ensure that the instrument was operating as expected. This was followed by a 10-day "commissioning" run in early May, and then a 33-day initial science run before the spacecraft made its first quarterly roll in mid-June to keep the solar panels facing the Sun. Because the search method requires a minimum of three transits, only planets with orbital periods shorter than about 14 days were detectable from the initial 43 days of observations. In fact, all of the planets that were announced had orbital periods between 3.2 and 4.9 days.

Fair enough, but shouldn't Kepler have discovered hundreds of such planets during the first 43 days? It probably did, but the mission requires detailed follow-up observations from ground-based telescopes to confirm each planet discovery. This is necessary to weed out "false positives" -- stars that look like they may host planets, but are actually something else. A simple example is an eclipsing binary star, where one star periodically passes in front of the other just like a transit. If there is a bright star close to the binary, or just along the line of sight, it dilutes the eclipses so they look like they are caused by a smaller object like a planet. The follow-up observations involve measuring the wobble of the host star caused by the orbiting object -- the traditional method of detecting planets around other stars. If the orbiting object is a star, the wobble is enormous; If it's a planet, the wobble is tiny.

Kepler is searching for planets in a large patch of the sky in the constellations Cygnus and Lyra. This area of the sky is well placed for ground-based telescopes during the summer months, but by the time the early data from Kepler was processed it was already August. The season for these follow-up observations was very short, so the team could only confirm a limited number of the planet candidates. In the end, they decided to announce only five of the more than 70 candidates that emerged from the initial analysis. By next summer, the mission will have many more candidates in longer orbits -- but they will also have a full summer season to confirm the discoveries.

By next January, expect to hear about hundreds of new planets. It will still be too early for the Earth-like planets, since that requires three 1-year orbits. But planets as tiny as the Earth may be found in faster orbits, and if they are hosted by smaller stars they might even be habitable. Stay tuned.