Studying the Chemistry in Protoplanetary Disks (Part I)

Today we have a guest post from Colette Salyk. Colette is the Leo Goldberg Postdoctoral Fellow at the National Optical Astronomy Observatory in Tucson, Arizona. She studies the evolution and chemistry of protoplanetary disks (the birthplace of planets) using a variety of ground and space-based telescopes.

One of the most interesting results to emerge from planet-hunting surveys is that planets and planetary systems are really diverse. I am trying to understand this diversity by studying the birthplace of planets – disks of gas and dust around young stars that we call “protoplanetary disks”. In particular, I study the chemistry in protoplanetary disks. In this post, I’m going to explain some of the techniques we use to detect and study molecules in protoplanetary disks using ground-based telescopes. In particular, I’m going to discuss the importance of the Doppler shift.

To detect molecules, we look for their unique spectral fingerprints. So we use spectrographs, usually on a large telescope like Keck or Gemini Observatory, or the Very Large Telescope. These observations require a lot of photons! But one challenge for these types of observations is that, if we’re observing simple molecules like water, carbon monoxide or methane, for example, these same molecules sit in the earth’s atmosphere and preferentially absorb the very photons we’re trying to detect.

This is where Doppler shifts come to the rescue. You may be familiar with Doppler shifts in the context of radial velocity searches for planets, in which the periodic Doppler shift in stellar absorption lines is produced by the gravitational pull of an orbiting planet. Recall that Doppler shifts are shifts in wavelength that are produced by relative motions between a source emitting photons, and an observer. If the source and observer are moving towards each other, the source looks blueshifted — its spectrum moves towards shorter wavelengths; if they are moving away from each other, the spectrum looks redshifted, like it has moved to longer wavelengths.

In our case, because both the protoplanetary disks and the earth are moving in space, the light emitted by molecules in the disk are seen at earth to be shifted in wavelength. Therefore, the wavelength of light we’re trying to detect is no longer exactly where molecules in our atmosphere want to absorb light.

The figure below shows an example of this. The red line shows the percent transmission of light through the earth’s atmosphere as a function of wavelength, as observed at the top of Mauna Kea. Note that at some wavelengths, the transmission is significantly less than 100%, meaning that the atmosphere absorbs a significant fraction of the light it receives from space. These regions are where water vapor molecules in the earth’s atmosphere are sucking up photons. In black is a spectrum of emission from a protoplanetary disk, obtained with the TEXES spectrograph on the Gemini North telescope. The peak in this spectrum was emitted by water vapor molecules in a protoplanetary disk. Note that it’s shifted relative to the sky absorption line due to the Doppler shift. In this case, the shift of the source line relative to the earth is consistent with a relative velocity of 18 km/s (∼40,000 miles/hour).



Spectrum of emission from a protoplanetary disk (black), obtained with the TEXES spectrograph on the Gemini North telescope. The peak in this spectrum was emitted by water vapor molecules in a protoplanetary disk. Image credit: Colette Salyk

This Doppler shift wasn’t just obtained by chance. Because it’s the relative motion of the earth and the disk that determines the observed Doppler shift, this shift actually changes throughout the year, as the earth orbits around the sun. The diagram below is a schematic representing a top-down view of the earth’s orbit, with the location of the Earth at four hypothetical dates, as well as a possible location on the celestial sphere of a protoplanetary disk. Note that while the Earth orbits the sun at a nearly constant speed, the direction of its velocity (represented by the arrows) changes. So there are times of the year when the spectrum of this protoplanetary disk is shifted towards longer wavelengths, other times when it is shifted towards shorter wavelengths, and times when it is not shifted at all.

Assuming the geometry in this schematic, what time(s) of year might you expect the Doppler shift shown in the first figure? When do you think would be the ideal time(s) of year to plan observations of molecules in this disk? When would be the worst times of year?


Image credit: Colette Salyk

Once we detect the molecules, what do we learn from them about planet formation? I’ll discuss this in more detail in a future post. But here’s some food for thought. The architecture of the solar system has a very clear division between terrestrial planets (at 1.5 AU and within) and giant planets (at 5 AU and beyond). What might have caused this dichotomy, and should we expect to see it in exo-planetary systems as well?

This is Not a Simulation

When someone tries to explain the concept of planet transits, you often see simulated graphics like the one below:

Well James Gilbert from the University of Oxford took the video of the transit of Venus he captured in 2012 (which was a real example of the transit technique in our backyard) and made the simulation into a reality. He measured the Sun’s brightness throughout the duration of the transit video, and  the result is the below:

That’s no simulation! It’s truly amazing. You can find out more about how James made the video and took the footage at his blog: LabJG: James Gilbert.

The Search for Supereccentric Jupiters

Today we have a guest post from Bekki Dawson. Bekki is a Miller postdoctoral fellow at the UC Berkeley Department of Astronomy. Her research focuses on how planetary systems form and evolve.

The origin of “hot Jupiters,” giant planets orbiting extremely to their host stars, remains a mystery. There are two major theories for how these planets “migrated” from a location like our own Jupiter’s to the close-in orbits we observe today. The first is gentle disk migration, in which the disk out of which the planet forms pushes the planet towards the star. The second theory is more violent: another body in the system perturbs the Jupiter onto a very eccentric (elliptical) orbit. Over time, tides on the planet cause dissipation that shrinks and circularizes the planet’s orbit. In 2012, Aristotle Socrates and collaborators predicted that if this secondary theory is correct, we should find half a dozen “supereccentric” Jupiters in the Kepler sample: Jupiters that are still on very elliptical orbits and have not yet tidally circularized.

To search for these supereccentric Jupiters in the Kepler sample, John Johnson and I developed an approach we call the “photoeccentric effect” to identify eccentric Jupiters from the Kepler photometry. The approach hinges on the fact that a planet transiting on an eccentric orbit will be moving at a very different speed than a planet transiting on a circular orbit with the same orbital period, leading to a different shape and duration for the transit light curve:


Even taking into account various degeneracies, we can easily identity the supereccentric Jupiters from their Kepler light curves. But surprisingly, we didn’t find any, inconsistent with the prediction. Therefore hot Jupiters continue to be a puzzle! However, the expected number was based on the number of planets that transit at least three times in the Kepler data, so in the future we hope to measure the eccentricities of giant planet candidates with only two transits; if those are missing too, the evidence would be even more compelling. Unfortunately, giant planet candidates with only two transits are not automatically caught by the Kepler pipeline, so if you spot any, let me know!

Planet Hunters at the Citizen Science in Astronomy Workshop

Last week, I co-hosted and co-organized the workshop on Citizen Science in Astronomy in Taiwan at my home institute, the Institute of Astronomy and Astrophysics at Academia Sinica. The aim of the workshop was to bring together  scientists from astronomy-based citizen science projects, computer scientists, and web developers to spend a week working on the problems and issues that  we jointly have in common when processing citizen science data and looking at how to improve these methods in the era  of petabyte datasets that is just around the corner with facilities like the Square Kilometer Array (SKA) and the Large Synoptic Survey Telescope (LSST) being built in the coming decades. In addition to Chris Lintott and myself representing Planet Hunters, several members of the Zooniverse development team and people from the science teams of Galaxy Zoo, Space Warps, Moon Zoo, Radio Galaxy Zoo, Planet Four, and the Andromeda Project were participating in this workshop. I had some really productive conversations and input on where to take the current analysis for the Planet Hunters data reduction that I’ve been working on in the past few weeks and months. All and all, it’s been a tiring but fun week.

The video and slides from the invited talks from the first day of the workshop are available online. Chris gave an overview talk on the Zooniverse and Citizen Science in Astronomy:

Stuart Lynn gave an overview of the technical side of the Zooniverse:

I gave a talk giving an overview of Planet Hunters:

Meet Our Talk Moderators

As many of you probably know, we have three moderators who volunteer their time to help the Planet Hunters community on Planet Hunters Talk . If you have questions, issues, or the rare dispute on Planet Hunters Talk,  Jo, Joe, and Tony are a great resource. You can  contact them directly via private message or hit the report button to alert them to a thread or post that requires their attention.

Let’s learn more about them:

Jo Echo Syan (echo-lily-mai)

Hi Planet Hunters. My tag is echo-lily-mai, many people wonder what to call me? Well, Echo is fine and some call me Lily. I’m very honoured to be moderator on PH and to be part of such a ground-breaking project.

I remember being in a room in Oxford England with other zooites (zooite is a pet term for people working on a Zooniverse project ) when the Planet Hunters project was announced. I can remember being seriously enthusiastic about the idea.

Planet Hunters Talk Moderator Jo Echo Syan

Planet Hunters Talk Moderator Jo Echo Syan

Here are some photos of us ‘zooites’ at Astrofest this year.

Image credit: Jo Echo Syan

Image credit: Jo Echo Syan

Image credit: Jo Echo Syan

Image credit: Jo Echo Syan

Over the years, I have worked on a few other Zooniverse projects. Galaxy Zoo which I am very fond of, and as a moderator on the now sadly retired Merger Zoo project.

I am interested in Art and Science and try to explore the two subjects through my Enjoy Chaos pages.

There are some amazing projects PH folks are working on at the moment, and I feel very proud to be part of the team when Planet Hunter papers are published.

This has only been possible because each and every person took part and helped with the project, whether that culminated in finding a planet candidate or not!!

I hope that I can help along the way, even if it is directing someone new to a link where they can find out what an eclipsing binary is (yep they look amazing) Or, by passing information on to the science team that needs to be checked out.

If you are new to PH, do ask questions, feel free to explore, and behave!!!! You can always contact a moderator if you have any concerns. Most of all enjoy.

We are all very lucky to be part of this science community, which a few years ago never existed and wouldn’t have been possible.

Is our Earth special? Of course it is. Is it unique? I hope we find out one day…

Tony Hoffman (TonyJHoffman)

My name is Tony Hoffman, and I’ve been fascinated by the night sky since I was an adolescent. Over the years I’ve participated in a number of citizen science astronomy projects, including the SOHO comets program; the Spacewatch FMO Project (near-Earth asteroids); Ice Hunters (Kuiper-belt objects), Stardust@home (interstellar dust); GalaxyZoo; and SETI@home; and have even had some success in finding new astronomical objects in several of them.


Planet Hunters Talk Moderator Tony Hoffman – The photo is of me in Kenya, about to find out that eclipses and dust storms don’t mix!

When Planet Hunters was launched, I shifted my focus to it, as the search for planets orbiting other stars is one of the great quests of our time. Up until I was in my 30s, there were no known exoplanets. In just the past 2 decades, a profusion of planets and solar systems have been found. The idea that ordinary people such as myself can take part in this endeavor staggers my imagination, and the success of Planet Hunters—in which a group of people “eyeballing” light curves have been able to find planets that eluded the Kepler project’s own search algorithms—has been a wonderful vindication of the idea that the human eye is better at some forms of pattern recognition than machines. It’s been a thrill to play an active role in Planet Hunters, and to have contributed to the discovery of at least one new world. I’m glad that Planet Hunters has been able to play a role in helping to survey what sort of worlds are out there, and how other worlds and solar systems are like or unlike our own.

I live in New York City, and I work as a writer. I’m glad to be able to help chronicle some of this great age of discovery. Being involved as a moderator in Planet Hunters has given me a personal connection to the science of exoplanetology. I’ve encountered some brilliant people whose skill at analyzing transits far exceeds my own. Although only a small fraction of the 200,000 (??) Planet Hunters volunteers may get their name on a paper or receive any formal recognition, everyone who classifies transits has an important role as a node in this vast human “computer” that can take graphs of a star’s brightness and find new worlds within them

Joe Constant (constovich)

Hello Citizen Scientists! I am Joe Constant and I live in South Carolina, USA with my wife and two beautiful daughters. From a young age I daydreamed about far away and fantastical places. Planet Hunters allows me to look skyward and potentially find some. We live in an exciting time, with the potential close at hand to answer one of humanity’s oldest questions – “are we alone?” The only time I would rather live in that now is in our future where our transportation technologies advance to the point to allow us to reach the distant rocks we are only now able to see.

Planet Hunters Talk Moderator Joe Constant

Planet Hunters Talk Moderator Joe Constant

When I am not on Planet Hunters or with my family, you’ll likely find me helping convert neutrons to electrons. I work at Duke Energy’s Catawba Nuclear Station coordinating the cause analysis program. Through this program we learn why adverse conditions occur (e.g., why did this pump stop working?) at the plant in order to improve performance and produce nuclear safety (electricity is just a by-product). If there is anything I can do to help, just let me know!

An Introduction to MAST

Today we have a guest post from Scott Fleming. Scott is a scientist at the Space Telescope Science Institute, located in Baltimore, MD, USA, where he works on the data archives.  His research interests include eclipsing binaries, stellar astrophysics, brown dwarfs, and extrasolar planets. Today Scott is here to tell you a more about MAST, the online public data archive where the Planet Hunters team obtain the Kepler light curves that are processed and eventually show up on the Planet Hunters site for you to classify.

The Mikulski Archive for Space Telescopes (MAST) is the official archive of data from NASA-funded space telescopes.  We primarily house data from ultraviolet and optical space telescopes.  Some of the missions we support include GALEX, the Hubble Space Telescope, the James Webb Space Telescope (after it launches), and of course, Kepler.  We also archive, and plan to archive, data from many other missions that have launched over the past 40 years, ranging from the 1970’s to future space and ground-based telescopes.

 Our role in the Kepler mission is to serve as the Data Management Center.  This means that, in addition to some data processing, we archive the lightcurve data itself (the timestamps, fluxes, flux uncertainties), as well as information related to each observation that’s required for calibration purposes, and catalog information that contains data on the host stars (their brightness in different wavelengths, estimates of their temperatures and sizes, etc).  We primarily serve professional researchers by facilitating access to the data, enabling powerful search capabilities so they can locate the data they need for their research, and providing tools that allow the scientists to preview and visualize the data before they download it to their machines for further analysis.  However, we do have some online tools that are used by educators and amateur astronomers as well.

Our newest tool is the MAST Discovery Portal.  This online search interface allows users to enter coordinates or target names and do a search for data across many missions all at once.  This is kind of like a “Google” for astronomical data, where users can discover observations that may have been taken on their objects, even if they weren’t aware of its existence beforehand.   You can enter the coordinates or name of a Kepler star, for example, and discover what other data exist by searching “All MAST Observations” or the “All Virtual Observatory Collections” in the top-left menu.  The Virtual Observatory is an online service that provides access to data from other astronomical archives around the world.  This allows users to search not only the ultraviolet and optical data at MAST, but also data in the radio, infrared, x-ray, and gamma-ray.


MAST Discovery Portal Astroviewer (image credit: Scott Fleming)

The Discovery Portal includes an AstroViewer.  Using background images of the sky created from ground-based surveys, users can see the “footprints” (i.e., the field-of-view) of a given piece of data, and see exactly where your objects lie inside.  If you’d like to try it out, do a search on “Kepler 2”.  In the AstroViewer on the right-hand-side of the screen you will see lots of footprints appear.  The small squares around stars in the field are from Kepler; they show which stars Kepler looked at in the field.  Our target, Kepler 2, is automatically centered in the AstroViewer.  You will notice larger squares around it, which are the footprints of data observed with the Hubble Space Telescope.  If you zoom out to larger scales using the “minus” button on the lower-left corner of the AstroViewer, you will start to see very large squares and circles.  The biggest squares come from the Swift space telescope, while the large circle is an observation from the ultraviolet GALEX space telescope.  You can see how this visualization of data from many missions allows users to discover new data on their targets, and look for cross-mission overlap that can enable new kinds of science when multiple instruments observe the same target.

 Feel free to try out the Discovery Portal for yourself.  There is no registration or login required.  You can follow MAST online on Facebook and follow us on Twitter @MAST_News.  Although our posts are directed at professional astronomers to alert them when new data and tools are available at MAST, it’s still a way to keep up-to-tabs on what new projects are happening in the professional astronomy circles.

15 percent of 1 million


The Zooniverse achieved a major milestone last Friday. The 1 millionth (that’s right 1 with 6 zeros after it!) person registered for a Zooniverse account.  While writing this blog, I decided to go and read the very first blog post formally announcing the Zooniverse. It’s amazing to see how far the Zooniverse has come and see the interesting science and exciting discoveries that have been produced as a result: starting with Galaxy Zoo launching in 2007 to  Planet Hunters launching in 2010 (where we were the 10th Zooniverse project)  to the Zooniverse  today now with over 20 projects spanning not just astronomy but  biology, ecology, history, and more.  Congratulations to all involved!

If you’re interested in seeing how those 1 million volunteers are distributed,  Rob Simpson has created a global map of the Zoonvierse community. Also Grant Miller and Rob Simpson have come up with some other cool graphics and interesting statistics about the Zooniverse on the eve of the 1 millionth registered volunteer. Check them out here, here, and here. Also if you’re interested in seeing all the scientific publications from Zooniverse projects (including the 6 published Planet Hunters papers), you can find them all on the Zooniverse publications page.


Image credit: Brooke Simmons

This accomplishment is yours too. Our estimate is over 280,000 people world wide (unregistered and registered volunteers) have participated in Planet Hunters. 150,000 registered Zooniverse volunteers have classified at least one Planet Hunters light curve. Roughly 15% of the 1 million registered Zooniverse volunteers have contributed to Planet Hunters science. That’s a huge representation! Thank you for the time and effort you put into Planet Hunters. We appreciate the time and effort you put in to help make the science happen. Thanks for being part of the Planet Hunters community and the larger Zooniverse community.

So let’s celebrate this milestone in the only way that seems  fitting:  classify light curves today at or maybe (just for today ;-) ) even take a look at some of the other Zooniverse projects at

May the Zooniverse Live Long and Prosper!

Some More Frequently Asked Questions

It’s been awhile since we’ve had a  blog post addressing some of the repeated questions we get from new volunteers that our Talk moderators, the science team, and other members of the Planet Hunters community have answered, so I thought I’d spend this week’s blog on this topic. You can check out our previous FAQ post from a long while ago here.

Q. Why can I only see 30 days of the Kepler data on the Planet Hunters classification interface but after I classify through Talk I can see more data?

A. Partly that is because of how the site was originally designed and how much data we had at the time.  We designed the Planet Hunters interface to show data from Quarter 1 which was roughly ~33 days long. When longer quarters of Kepler data were released, we thought that showing the full Q1 data in one go was working well for Planet Hunters, so we decided to cut the longer observations  into smaller sections that we would send to different volunteers to classify. Each 30 day light curve segment receives 5-10 independent volunteer assessments. On the Talk page for the light curve you can go to examine star and that will take you to our source pages that show all the light curve sections available for that star in the Planet Hunters database that you can scroll through and zoom-in like in the Planet Hunters interface.

Q. After classifying more stars on Planet Hunters, I think I want to change my answer to a previous classification. Can I change it?

A. No, once you’ve submitted your classification for a given light curve on Planet Hunters there is no way to go back. We want your first opinion without  being influenced by others or added information. For example, you might mark more transit boxes on the light cure of  a star that you learned from Talk is a Kepler planet candidate than for a light curve of a star that you knew wasn’t a Kepler planet candidate. So we don’t allow you go back to change a classification after it has been submitted or know about the Planet Hunters ID of the star until after you have classified.

Q. When is new data uploaded to the site?

A. Light curves are uploaded once we’ve finished a Kepler Quarter (which is 90 days of observations). There is roughly 160,000 stars that get monitored each quarter (the same ~160,000 stars are watched from quarter to quarter by Kepler during its prime and extended mission), and we chop those light curves into 30 day segments. So that’s what you’re seeing on the site, it’s not the first 30 days, it’s currently one of three sections of Quarter 16 of Kepler observations.

Q. Do you announce discoveries?

A. Yes, we announce  the discovery of new planet candidates and science papers on the blog and all of the scientific papers resulting from Planet Hunters classifications and Talk discoveries can be found here.

Q. Are you giving  credit to those that find the candidates?

Yes we are absolutely giving credit to people who are identifying planet transits that become discoveries. Each published paper from Planet Hunters has an acknowledgments section where we list the people who contributed to the finds. We also have these acknowledgement sections as websites. If the planet candidate is a significant component of the paper then we may add the discoverers as coauthors to the paper. You can check out the published papers here. We also acknowledge all Planet Hunters volunteers here.

Q. What about this single low points (see example light curves below) ? Should I mark those as transits?

APHF1140285 APHE1001w3h

A. No, these single low points you see in the two example light curves above are not transits. They are likely just bad/spurious data points. If you see a single low point don’t mark it as a transit. Transits typically last a few hours to tens of hours, so there should be more than 1 low point if there is a planet transiting the star.

Q. Aren’t there automated ways with computers that you could search for these planet transit signals?

A. Yes, and the Kepler team and many other astronomers are doing just that. There are automated methods that have been developed and are being used to look for transits in the Kepler data. Many groups of astronomers are looking through the Kepler light curves using computer programs that look for repeating signals due to transits. The Planet Hunters team thinks that there may be transits missed by the computer that the human eye may find so that’s why we started the project. We have found planet candidates missed by the automated routines. You can find out more on the results from the project on the blog. If you want to try your own analysis, you could start by using the csv file of the light curve data we provide on the source pages (click on examine star on the Talk page of the light curve), but if you plan on doing a full analysis you’ll want to get the data from the NASA public archive MAST at

Q. What’s a Kepler Threshold Crossing Event?

A Threshold Crossing Event or TCE is a potential transit event  identified by the Kepler team’s automated computer algorithms during a search of  Kepler data. The majority are false detections, but a few are real transits due to orbiting exoplanets. A subset of the Kepler team examine the TCE list and whittle it down with other checks and analysis to make the Kepler planet candidate list. We mark the light curves of stars where the Kepler team has detected a TCE on Talk with the  label ‘Kepler Threshold Crossing Event Candidate’.

If you have other questions, check out our FAQ site or Site Guide. If you don’t see the answer you’re looking for, do ask your question on our Talk website

Sky & Telescope

Some time ago, I was asked to write an article for Sky & Telescope Magazine about Planet Hunters, citizen science, and how the public can contribute to science with the Zooniverse. The new edition (March) of Sky & Telescope is out, and my article (‘How You Can Find An Exoplanet‘) is  featured on the cover.  I’m really pleased with how the article turned out, and I especially  love the cover slogan: ‘Planet Hunting Goes Public: No Sky, No Scope, No Problem!’  Maybe we should adopt that as the official Planet Hunters motto. What do you think?

You can read more in the digital version if you have a subscription or in the printed magazine which should be out on newsstands soon. There is also a digital companion piece to the article that I wrote on Planet Four and Space Warps which is freely available online here if you care to check it out.

What Do We Really Understand About Planetary Formation?

It’s fair to say that there are a lot of gaps to fill in our knowledge of exoplanetary bodies, and 2013 proved to be a good year for bizarre discoveries. From a planet found with no star in sight, to a gas giant orbiting at an unfathomable distance, to a system containing an orbital plane 45 degrees out of whack, the list seems endless. As 2014 kicks off, we can expect no slowing down of these unusual discoveries from planet hunting teams around the world, on top of the months of Kepler data still queuing up for analysis. This opens up the topic of how these extraordinary bodies came to be, and begs the question, ‘what do we really understand about planetary formation?’

First of all, we need to imagine the early disk environment around a newly born star. The protoplanetary disk contains lots of dust and gas left over from the initial collapse of the interstellar cloud from which the star forms. Both the star and the disk rotate about a common centre of gravity, and it is the rotating debris, ranging in size from an angstrom up to a centimetre, that can evolve in the disk to form planets.

Artist Impression of Planetary Formation, courtesy of NASA

Artist Impression of Planetary Formation, courtesy of NASA

There are two widely held theories for how giant gas planets can form: core accretion and disk instability. Core accretion occurs from the collision and coagulation of solid particles into gradually larger bodies until a massive enough planetary embryo is formed (10-20 Earth masses) to accrete a gaseous envelope.  Disk instability, on the other hand, describes the process by which a massive disk rapidly cools, causing it to fragment into planet-sized, self-gravitating clumps. Both theories can be used to define the presence of giant planets, but there are a few pitfalls in these explanations and a plethora of planets that neither theory alone can seem to justify. Let’s look in more detail.

The primary accepted mechanism of planet formation is our first theory, core accretion, which is best described in several stages. During the first step, material in the disk collides and aggregates to form small centimetre to metre sized clumps of matter. The clumps then grow further by smashing and sticking together, leading to the gradual coagulation of kilometre-sized planetesimals. Some of these large bodies are massive enough that runaway accretion begins, resulting in the rapid formation of planetary embryos. Here there is a distinction between the formation of terrestrial and gaseous planets. Near the star, heavier metallic elements begin to condense at hotter temperatures and violent collisions and mergers can eventually result in the production of terrestrial planets. The bodies remain relatively small due to the amount of material found in the inner disk, and explains why the terrestrial planets in our solar system lie closest to the Sun. Farther out from the star beyond the snow line, embryos form from a mix of rocky, metallic and also a considerable amount of less dense icy material. At such cool temperatures, hydrogen and helium are able to condense and build to form much larger bodies. Around 10 Earth masses, the planet then possesses enough gravitational attraction to accrete a gaseous atmosphere of hydrogen and helium, a process which continues until all the gas in the planet’s vicinity is exhausted. This describes why the planets in the outer Solar System predominantly consist of lighter elements and are able to acquire such large atmospheres.

Artist Impression of Accretion. Image Credit: Alan Brandon/Nature

Artist Impression of Accretion. Image Credit: Alan Brandon/Nature

However, this mechanism struggles to explain massive planets forming at large distances from a star. This has led to HD 106906 b, whose orbit is 650 times greater than the Earth’s orbit around the Sun, to be proposed as forming independently from the star altogether! A problem closer to home is the extremely long time-scale required for Neptune and Uranus to form a core through accretion, which is estimated to be around 10 million years. Since the gas and dust in the protoplanetary disk probably only lasted for a few million years, this poses quite an issue. Newer accretion models may be able to account for their formation within a short enough timescale, but this is still a challenging and ambiguous area. Alternatively, could our ice giants have formed via a different mechanism?

The Solar System. Image Credit: International Astronomical Union

The Solar System. Image Credit: International Astronomical Union

A different theory of giant planet formation is via disk instability; a less popular, but still plausible, explanation. This mechanism requires no direct interactions between solids whatsoever, just the condensing of gas and dust in the planetary disk. During the very early stages of a protoplanetary disk’s formation, if rapid cooling occurs in the order of an orbital timescale, material is thought to fragment into bound objects. These fragments would then condense further into the gaseous planets we observe. This theory provides an explanation of planet formation that would occur within a very short (few thousand years) timeframe, and can also be used to explain the presence of large gaseous planets near to or very far from the star. However, whether a disk could cool quickly enough to fragment on an orbital timescale is hotly debated. It could be that it is only a possibility at very large orbital radii.

With two competing theories for how the most massive planets form, we still have a lot to learn about the evolution of the different systems we observe, especially our own! It is likely that the formation mechanism is dependent on the system, and that both theories could work within different regimes. But neither of these theories seems to explain the presences of hot Jupiters; gas giants that orbit incredibly close to their host stars with periods of just a few days. It is believed that at such close proximity to the star, temperatures would simply be too high for the planet to retain its gaseous envelope during formation, which is where the idea of planetary migration really came to light. This suggests that perhaps where we observe a planet now isn’t really where it originally formed at all.

Check back soon for my next post discussing the different theories of planetary migration.

Happy hunting!


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