Micro-resonator laser frequency combs and Exoplanets

This past week Caltech and the Keck Observatory published the following Press Release:

http://www.caltech.edu/news/new-calibration-tool-will-help-astronomers-look-habitable-exoplanets-49624

http://www.keckobservatory.org/recent/entry/new_calibration_tool_will_help_astronomers_look_for_habitable_exoplanets

Wavelength calibration is but one part of the multi-front battle in the quest to detect the velocity reflex motion of an Earth-massed exoplanet in a Habitable Zone orbit of a Sun-like star.  Such an exoplanet causes the star to wobble back and forth with a characteristic velocity of ~10 cm/s, a very slow walking speed!  

Other battlefronts in the quest to detect another Earth around another star include maintaining the stability of the starlight illuminating the spectrograph, the optical and mechanical stability of the spectrograph itself (pressure, temperature, etc.), the properties of the detector that measures the  spectrum, how one analyzes the data, and distinguishing the behavior on the surface of the star itself (starspots, granulation, p-mode oscillations) from the motion the star experiences from an orbiting exoplanet.  All must be dealt with in tandem, but for this blog post I will focus on wavelength calibration.

Wavelength calibration involves mapping a set of particular wavelengths of light to individual pixels on a CCD (aka, the detector or digital camera) in a spectrograph.   Emission line lamps provide a means for wavelength calibration.  Thanks to quantum mechanics, light emits at very specific wavelengths from gases of particular atoms or molecules, and helps explain things like why neon lights appear red.  A common lamp used in astronomy is made of Thorium and Argon, emitting light at a wide variety of colors across the visible spectrum of light.  By measuring a spectrum of a Thorium Argon lamp with the same spectrograph that is used to measure the spectra of stars, astronomers could get a rough idea of what wavelengths corresponded to what pixels on the CCD, and apply that same relationship to the spectra of stars.  Then as stars moved towards us and away from us from the gravitational influence of an orbiting exoplanet, the spectra of the stars will shift in wavelength every so slightly to the blue or red.  By carefully measuring the wavelength shifts, using the Doppler Equation we can solve for the velocity of the star, after accounting for the fact that our telescope is also moving along with the Earth in its orbit in our Solar System.

The revolution in the discovery of exoplanets orbiting other stars was wrought through a break with tradition and advances in technology.  

Astronomers have used high-resolution spectrographs for decades. A high-resolution spectrograph can split light into many different colors; mathematically, this is represented as R = lambda/(delta lambda), where lambda is the wavelength (specific color) of light, delta lambda is the smallest two wavelengths one can distinguish, and R is the spectrograph resolution. A typical R value is 100,000.

With the first confirmed exoplanet orbiting a main sequence star found (and commonly accepted) in 1995, why didn't astronomers find exoplanets decades earlier?  The reason is three-fold: The invention and steady improvement of CCDs, changing how we perform wavelength calibration, and accepting that spectrographs are dynamic instruments - flexing with changes in orientation, pressure and temperature.   Traditionally, many spectra of stars would be obtained over the course of a night, and at the beginning and/or end of a night, astronomers would take observations of emission line lamps like Thorium Argon.  The spectrograph was assumed to be "ideal" over the course of the night (stable and unchanging), and the same wavelength solution would be used for all of the data on a given night.

This assumption of stability was an over-simplification.  As a result the best precision that could be obtained for measuring the velocities of stars was on the order of 1 km/s.  1 km/s  is comparable to c/R, where c is the speed of light, and R is again the spectrograph resolution.  This was five orders of magnitude worse than what was needed to detect another Earth.  In order to make progress, this assumption needed to be abandoned.  Astronomers took two approaches that both worked for finding exoplanets in the mid-1990s - using alternative sources for wavelength calibration that could provide simultaneous illumination of the spectrograph, such as the iodine cell technique, and stabilizing the spectrograph by making it as "ideal" as possible.

Fast forward to today, and spectrographs are built with extreme stabilization and purposefully built for the detection of exoplanets.  Thorium Argon lamps are still used today for the HARPS and HARPS-North spectrometers, arguably the best performing spectrographs for exoplanet detection in the world.  The teams of astronomers behind these instruments have recognized that the lamps can limit their ability to search for other Earths, reaching a precision of ~70 cm/s, now only one order of magnitude shy of what is needed.  In order to make future progress on this battle-front, a new wavelength calibration method is needed.

Enter the laser frequency comb.  

Originally invented for increasing bandwidth for telecommunications hardware over fiber optics cables, the devices pulse laser light at a very high fixed repetition rate such that the resulting spectrum is a series of sharp emission lines.  Hence the word "comb".  In wavelength space, the spectrum looks like a comb:

A laser frequency comb spectrum

A comb

Physicists at NIST realized they would make great wavelength rulers, and developed the technology to stabilize them using a variety of techniques.   Once the laser comb spanned an octave (a factor of two in wavelength), the use of a frequency doubler allowed the comb to be "wrapped around" in wavelength and reference one end of its spectrum to the other end of its own spectrum, a process called "self-referenced mode locking."  The resulting stability of the wavelengths of the laser comb emission lines was unprecedented, measured to be better than 1 part in 10¹⁵.   The possible application of laser frequency combs to astronomy in general and exoplanet searches in particular was obvious.  Here was a means to wavelength calibrate a spectrograph to much better than 1 cm/s.  But there was a problem - the emission lines were too closely spaced together.  So, physicists at NIST developed the hardware to filter out most of the lines - 49 out of every 50, or 99 out of every 100 using two Fabry-Perot etalons in series - leaving just enough lines that an astronomical spectrograph could tell them apart.  These filtered laser frequency combs have now been tested on HARPS and HARPS-North, as well as other spectrographs, and the results are incredibly promising.

The catch - it takes a team of well qualified physicists to run them, along with $1 million in hardware.  Today you can buy one of these combs from Menlo Systems, if you've got a cool million to spare.  With modern astronomical spectrographs now costing $5-$30M apiece, an extra $1M is a lot, but not totally unreasonable.

Enter the micro-resonator laser frequency comb. 

So, the physicists at NIST started working on new technology to build combs that natively have wider spacing between emission lines, called a micro-resonator.  It would simplify the hardware, and perhaps lower the costs.  Collaborating with Kerry Valhala and graduate student Xu Yi at Caltech, scientists at NIST built a prototype that fits on a moderate sized optical breadboard (~24"x24").  But would it work on a telescope?

That is where I come in.  In the press release, I'm listed at the end as "other authors of the paper" for my five seconds of fame:

"Peter Plavchan (BS '01), formerly at Caltech and now a professor at Missouri State University;"

Yup, not only did I get my Bachelors degree in physics from Caltech, but after earning my PhD from UCLA in 2006, I spent another 8 years as a postdoctoral scholar and research scientist at Caltech (JPL and the NASA Exoplanet Science Institute in particular).   Those 8 years of effort and a lot of elbow grease are going to have to suffice in place of any future alumni donations!

It was a great experience to be part of this project bridging two fields - Astronomy and Applied Physics.  My contribution was providing the hardware and the expertise for getting first light with the micro-resonator comb at the NASA IRTF telescope with the CSHELL spectrograph. We had simultaneous star and laser comb light going down to the spectrograph via fibers from one of my instruments.  Basically, I built part b in the diagram below back in 2012, which I published a paper on in 2013:  

Figure 2 from Xu Yi's Nature Communications paper

In 2013 and 2014, I helped design and build a custom part that place two fibers - one carrying starlight and the other carrying the micro-resonator laser comb light - very close to each other in the above diagram.  How close?  Let's just say it required dipping the tip of one of the fibers in acid, an X-ray imaging device, and a 3-D printed part, all courtesy of the engineers and scientists at JPL.

In July of 2014, I went out to IRTF atop Mauna Kea for a few nights for our first engineering run using the comb on the telescope.  It was only one month after my infant son Lincoln was born and after we had moved halfway across the country.  It's cold at the summit of Mauna Kea, nearly 14,000 feet above sea level.  And your brain doesn't work quite right, getting only about 60% of the oxygen you would get at sea level.  Since it was my instrument that we were adding this new dual-fiber part to, I had to be there to train the others on the team with how it worked.  Unfortunately while we were mounting the two bare fibers into the hardware, the fiber broke clean off.  The fibers weren't protected by a "sleeve" and thus were very fragile.  As a result, we could only put unfocused light from the microresonator comb down into the spectrograph.  At least we verified that worked, and this gave us confidence to come back and try again. The comb was working fine, but we identified a few ways to improve it.  And we needed to repair the fiber.

By the time we got more engineering time on IRTF in September 2014, I was in Pasadena for my last three days before starting my job as an assistant professor at Missouri State University.  I called in remotely via Skype to help with the commissioning.  We brought the comb back out to the telescope, this time with some further improvements.  Other members of our team that had gone in July were back on site atop Mauna Kea.  This time we had protected the fibers from breaking, and it worked!

Figure 4 from Yi et al. 2016.  In the upper right, you can see two simultaneous spectra.  The top spectrum is from the micro-resonator laser frequency comb, and the bottom is from the starlight of SV Peg, a bright K=0 red giant. 

Unfortunately, in September 2014 the weather was so bad, we barely collected a few hours worth of data at night time.  We had to look at the very bright red giant star SV Peg, because clouds were reducing the apparent brightness of stars by over a factor of 100!  So, the radial velocity measurements were not ideal.  But we demonstrated that the technology was feasible, repeatable, and that the spectrum of the microresonator comb could be captured simultaneously with starlight.  Several months later we would successfully test it again on a different spectrograph during the daytime - the NIRSPEC spectrograph on the Keck Telescope (one of the largest in the world) - showing the versatility of the comb and how "turn-key" the device was after months of being in storage.

From the NIRSPEC tests, we learned that the micro-resonator comb is at least good enough to measure radial velocities at a precision of 30 cm/s, with room for improvement in the future.  Currently, it's only limited by the stability of the reference laser we use to "lock" the wavelengths of the comb.  In the future, we may be able to get the device to span an octave in wavelength, and thus obtain a "self-referencing" lock, which will hopefully be stable to better than 1 cm/s.  With time, we believe this device will be more cost effective, and less complex, enabling routine use of the micro-resonator comb for the wavelength calibration of spectrographs in our search for another Earth.  During our last test it only required one highly skilled applied physicist to set it up, and we envision a future where these will be as easy to use as Thorium Argon lamps are today.

Here's the full content of the scientific paper describing our results:

http://www.nature.com/ncomms/2016/160127/ncomms10436/full/ncomms10436.html

Planet X vs. Nibiru in the news; when pseudo-science hijacks genuine science, and how to tell the difference

This week a proposed new planet far out in our Solar System was announced in the news.  I was asked by a non-scientist:

Okay, so as a newbie to astronomy, does this Planet of Niburu aka Planet X exist?

Their email included a link to this blog post from live.slooh.com:

http://main.slooh.com/event/is-newly-discovered-planet-nine-the-lost-planet-of-nibiru/

I'd never heard of Slooh.com, and the blog post started off innocuously enough:

Researchers at Caltech announced on January 19th they had discovered evidence of an as-yet unseen giant planet, 10 times the size of the Earth, which may be tracing a bizarre orbit requiring 10,000 to 20,000 years to make a revolution of the Sun.

CalTech researchers, Konstantin Batygin and Mike Brown, made the discovery using mathematical models and computer simulations. The planet itself has not yet been viewed through a telescope, but they hope their new paper on the discovery will inspire astronomers to attempt to find it. They say Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt.

"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there," says Batygin, an assistant professor of planetary science. "For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."

This much is true (well, I wouldn't have the used the word "mysterious" but "peculiar").  I had read the Nature News article summarizing the recently published scientific paper, which goes through a peer review process before being accepted for publication:

http://www.nature.com/news/evidence-grows-for-giant-planet-on-fringes-of-solar-system-1.19182?WT.mc_id=TWT_NatureNews

and the actual scientific paper, containing all the lovely jargon and math astronomers need to specifically and precisely communicate their findings:

http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22

But then the slooh.com blog post switched gears into the realm of pseudo-science in a way that a non-astronomer would have trouble noticing:

This isn't the first time that someone has suggested that an unknown giant planet could exist beyond what we consider the edge of our Solar System. It's just the first time scientists have found their own evidence to support any version of that claim. In his 1976 book, The 12th Planet, Zecharia Sitchin, a proponent of theories of ancient astronauts, claimed that Sumerian and Babylonian mythology supported the idea of another planet orbiting well outside the reach of the planet Neptune. He named this planet Nibiru, and claimed that every 3,600 years, the planet's orbit brought it past Earth so its inhabitants could interact with humanity.
In 1995, Nancy Lieder, a self-described "contactee", warned the world about an impending cataclysm. Lieder claimed that Nibiru, which she originally referred to as Planet X, was doomed to collide with Earth, causing an apocalyptic event that would flip the magnetic poles of the planet. This cataclysm was originally predicted to occur in 2003, but was delayed to coincide with the Mayan Calendar conspiracy of 2012, then delayed again. Sitchin has denied claims that Lieder's planet and his are one and the same.

So, to a trained eye like my own, there is definitely pseudo-science mixed in there.  To be fair, the article does go on to address this "controversy" and acknowledges Nibiru is a "figment of imagination" and this new discovery is "supported by science":

Astronomers, though, have long disputed the existence of Nibiru, and have insisted that there is no threat to Earth. In fact, Mike Brown, one of the authors on this brand new paper, has been one of the most vocal opponents of what he refers to as "pseudo-science". Brown has said that a planet with an orbit like that which is assumed about Nibiru would only have lasted about a million years in our Solar System before being ejected by Jupiter, and that the idea that any planet could force a flip in Earth's magnetic field defies the laws of physics.

But conspiracy theorists won't back down from their beliefs, holding fast to the claim that NASA continuously denies proof of the existence of Nibiru. Most of these involve claims that Nibiru has actually been observed in a variety of telescopes and sky surveys, but that evidence of those observations have been redacted.

During our live broadcast, we'll take a look at these two worlds -- one the figment of imagination, the other an unobserved world supported by science -- and discuss the coincidental intersection of science fiction and science fact.

This "Nibiru" planet has been fear-mongering people on the internet for at least a decade. People would post pictures of the overexposed Sun or Moon with internal camera reflections claiming to be of the Sun (or moon) and Nibiru, and occasionally Photoshopped:

Internal reflections in camera optics, also called ghosting or lens flares, are well understood and common. Here's some example web pages describing the effect:

https://photographylife.com/what-is-ghosting-and-flare

http://darkerview.com/CCDProblems/internalreflection.php 

This is not to be confused with another digital camera artifact that has been falsely associated with "Nibiru" as well, called saturation, like these over-exposed objects (probably Solar System planets) in this Sun monitoring satellite image:

Not planet Nibiru either.  Nor a UFO.

This ESO page has an excellent summary of common CCD (digital camera) artifacts seen in astronomical images:

https://www.eso.org/~ohainaut/ccd/CCD_artifacts.html
 

The Nibiru internet hoax reached a preposterous level in 2003, stoking fears on the internet of a planetary collision with Earth that were entirely unfounded.  Some UCLA solar astronomers posted this in front of their webcam on Mt Wilson when I was in graduate school at UCLA:

 

This was in response to some of the Planet X hoaxers using an image from the same webcam showing the moon and a crescent "Nibiru", which was in fact just an overexposed moon and an internal reflection within the camera optics:

 

As far as astronomers know, there is no undiscovered planet that is Earth-sized and on a highly elongated orbit that comes into the inner Solar System on a semi-regular basis.  Jupiter's gravitational influence would have ejected it long ago as Mike Brown states above.

Astronomers have looked for planets far out in our Solar System.  Why wouldn't we want to find one?  Moderate fame and "fortune" would await, right?  Maybe even a Nobel prize.  The NASA WISE mission, which mapped almost the entire sky at infrared wavelengths of light and was used to find many near-Earth asteroids, was able to rule out Saturn and Jupiter sized objects way out in our Solar System, except in the really dense stellar regions towards the plane of our Milky Way Galaxy.  In the plane of our Milky Way Galaxy, the field of view is so crowded with distant stars, it is difficult to see relatively nearby Solar System objects moving by. This study was published only two years ago.  Science moves fast!

https://www.nasa.gov/press/2014/march/nasas-wise-survey-finds-thousands-of-new-stars-but-no-planet-x/#.VqUAk_H19ks

And the scientific paper:

http://iopscience.iop.org/article/10.1088/0004-637X/781/1/4/pdf

The point is - we've looked!  In this study, the NASA WISE mission didn't have the sensitivity to see something like this new hypothesized Planet X, with a mass around that of Neptune.  It simply would have been too faint to have been seen with that space telescope.

This new possible planet is inferred indirectly from detailed simulations of the orbits of objects out near the orbit of Pluto called "Kuiper Belt Objects" which is where many of our comets come from.  Mike Brown is the second author on this paper, and he's also the scientist that "demoted Pluto" from planet status to dwarf planet status by discovering some of these Kuiper Belt Objects, including several that are similar in size to Pluto. 
  
The authors of the paper found that the orbits of a subset of these newly discovered Kuiper Belt Objects had curiously aligned orbits.  Much like the mapped orbit of Uranus led astronomers to predict the existence of Neptune, this new paper is claiming that the peculiar orbits of these Kuiper Belt Objects points to the existence of this possible new planet.    So, we haven't seen the new planet directly.  We've only got fairly solid evidence that it should exist.  We're not even entirely well constrained on what part of the sky it will be found in.  That will take more time and research.

This new hypothesized planet won't come closer to the Sun than about 200 times the distance between the Earth and the Sun (~100 million miles, or what astronomers define to be 1 Astronomical Unit or AU).  And we do see evidence of some exoplanets orbiting other stars that far out (http://exoplanetarchive.ipac.caltech.edu/).  So, the possible existence of such a planet is not entirely unheard of in other stellar systems.  And we know that the disk of material available to form a planet when a star is very young can extend that far out as well (https://en.wikipedia.org/wiki/AU_Microscopii). 

Why is this possible new planet and these Kuiper Belt Object discoveries just happening within the past 20 years?  Why didn't we find them 100 years ago?    

Well, the Kuiper Belt Objects are frozen cold "dirty snowballs" and made out of the same material that comets are made of. In fact, many of the comets that we periodically see in our inner Solar System get their start in the Kuiper Belt out beyond the orbit of Neptune. It was because of the orbits of comets that did come closer to the Sun that we knew this belt of objects must exist out beyond the orbit of Neptune as far back as the 1930s (https://en.wikipedia.org/wiki/Kuiper_belt).  In fact, the first discoveries of true Kuiper Belt Objects in the 1990s was a great triumph of the Scientific Method developing a theory that made a prediction that was testable and eventually shown to be true by repeated experiments (telescope observations).

Because Kuiper Belt Objects are frozen dirty snowballs and not stars powered by nuclear fusion, they don't emit much of their own light at visible wavelengths that we can see, but rather reflect sunlight.  The further away they are from the Sun, the less Sunlight they will reflect, and it falls off very steeply with distance from the Sun (1/D⁴).  So, these objects are very faint, and require searching large portions of the sky.  Put those two things together, and you need large telescopes, lots of data, and advanced software to handle processing and searching it all.  It is far too much to search by hand, and we rely on computers to do much of the tedious work for us today.  These objects also emit their own heat, which is what the WISE mission searched for at infrared wavelengths.  But again these objects are so far away they end up giving off very little heat because they're so cold. 
 

So, how can you distinguish science from pseudo-science?

Being a busy scientist myself, it's too easy for me to say "trust us."  For example, the nature.com website which regularly published peer-reviewed scientific papers is "trustworthy" whereas a "planet x" website is not (http://planetxnews.com/).  Such a logical argument is called an "appeal to authority".  And yes, while astronomers are authorities on astronomy, this is a weak logical argument to use.   Why should you trust my blog post over another?

The answer lies in the predictive power of the scientific method.   It is GOOD to be skeptical.   Scientists make predictions that can be tested.   So, test them!  Get educated enough to do the experiment yourself.  Because you DON'T have to take my word for it.  That is the beauty of science.

Proponents of the Nibiru planet did make predictions, and they failed again and again.  In 2003, in 2012. No mysterious Nibiru planet came swinging by the Earth to destroy us.  You would think by now one would recognize that their pseudo-scientific theory was wrong, rather than trusting their next prediction that will come to pass without incident.

But we can go further.   Drs Batygin and Brown predicted that we will find a new population of Kuiper Belt Objects whose orbits should follow a particular pattern, if this Planet X does exist.  And of course, we will look for direct images of this planet.  However, the current information doesn't give us enough data to pinpoint its location (yet), and it will be very faint.  So, we're going to look for these new Kuiper Belt Objects.  It will require more dedicated telescope surveys.  With time, the possible location of Planet X will become clearer, and maybe we'll find it.  Maybe it will take 50 years.  Maybe we won't find Planet X and maybe we'll find a better solution for the peculiar orbits of some Kuiper Belt Objects that does not require a Planet X (although unlikely).

Another pseudo-scientific claim that is often made is that NASA hides data or evidence of a Nibiru planet.  All NASA mission data is public.  NASA is funded by the public taxes.  Here is a link to the WISE mission data:

http://irsa.ipac.caltech.edu/Missions/wise.html 

You can find other mission data with a quick Google search.  There's nothing missing or hidden.  You can check for yourself.  One caveat is that you may need to educate yourself.  Ignorance is not an excuse.  If you genuinely want to know the difference between science and pseudo-science, you'll have to learn some of the science yourself in order to perform the experiments yourself.

So, the next time someone posts a photo of planet "Nibiru" next to the Sun, go outside the next clear day.  Is it still there?  Can you see it with your own eyes?  If not, why not? Wouldn't that be damning evidence against the Nibiru pseudo-science?  You don't have to take my word (authority), but do you really think thousands of professional astronomers would miss it, or conversely all work to cover it up?  And somehow a few internet websites with authors with little professional training and experience (who don't even understand the basics of camera optics) were able to find it?  Occam's razor suggests otherwise.  In the case of the "crescent Nibiru" seen on a UCLA Solar Observatory webcam back on May 26th, 2003, go ahead and use a lunar calculator online to figure out the phase of the moon on the night that web cam image was taken.  It matches up perfectly.  Here's a link: http://tycho.usno.navy.mil/vphase.html

Next, try taking your own pictures to reproduce the internal camera reflections that look like "Nibiru".  It might not be as easy with modern cell phone cameras that can have optical coatings to minimize internal reflections.  If you can create an internal reflection, you'll quickly see that what is in your digital image doesn't match what you see with your own eyes.  Try a different angle.  How about a different time of day?  Go to a location where the Sun is visible but any real "Nibiru" should be blocked by a object on the horizon.  If it's an internal camera reflection you should still see this effect.

In fact, I just took my iPhone 5S cell phone, pointed it at a light in my ceiling at an angle, and got this picture.  Is it 9 planet Nibiru's in my kitchen orbiting my light bulb?  No, it's an internal reflection of the over-exposed LED lights:

Not planet Nibiru either.

Science progresses through the ability to independently verify the claims of others.  Sometimes we as scientists fail to show you how to do this, and too often rely on asking you to take our word for it.   So, don't take my word for it. 

By the way, Dr.s Batygin and Brown have nicknamed the hypothetical new planet "Fattie".