Sagan Exoplanet Summer Workshop
Greetings from sunny warm southern California. I’ve been spending the week at Caltech in Pasadena, CA for the Sagan Exoplanet Summer Workshop. It’s been a full week of talks, tutorials, and hands-on session on the latest on transit light curves both in science results and analysis. There are about ~140 people mainly postdocs and graduate students who are working on or are interested in getting into studying exoplanets. The talks are geared for new people in the field with researchers in a variety of related subject areas talking about open questions and surveying where the topic currently is.
All the talk slides are online if you’re interested in seeing what’s been discussed. Also, the talks are being recorded and eventually will be posted online. There were also electronic posters (I submitted one for Planet Hunters) which you can peruse here and here. Also many of the participants gave POP talks which were short 2 minute talks which I think gave a great sense of the wide variety of people in attendance. I gave one trying to highlight everything we’ve done so far in the project is 2 minutes (it was tough to boil all of it down to 2 minutes).
It’s been nice to be back at Caltech where I went to grad school, but I’ve really enjoyed learning about different tools and techniques written to fit transits, estimate masses of planets from transit timing variations in the light curve, and process and analyze Kepler light curves (PyKE). Today is the last day of talks. I’m a bit sad to leave to the warm California sun, but I’m keen to bring the tips and tricks I’ve learned this week back to New Haven and apply them to the analysis of the Planet Hunters data.
~Meg
Electronic Poster
I wanted to share you the poster I’ve just submitted for the Sagan Exoplanet Summer Workshop. It’s a yearly workshop hosted by the NASA Exoplanet Science Institute located at Caltech, and the workshop will be held just a few buildings down the street from where I had my office in graduate school. The workshop is in a few weeks at the end of July. It’s a little different than a conference because it’s not necessarily to present your latest results, but bring together people to teach analysis techniques and gather exoplanet experts with graduate students, postdocs, and researchers who are involved in exoplanets as well as those interested in getting involved in exoplanet research.
The theme of this year’s workshop is “Working with Exoplanet Light Curves”. There will be talks and sessions about different topics related to analyzing light curves (including from Kepler) and there will also have hands-on sessions to teach new analysis techniques and software packages developed to study transits. I’m hoping to pick up a few new tricks to help with analysis and confirmation of Planet Hunters planet candidates and in general learn more about the methods other scientists are using to analyze light curves.
I’m also presenting an electronic poster (which will be shown in a rotation of other posters throughout the duration of the conference on computer monitors ) as well as a short 2 slide talk on what Planet Hunters has been up to over the past year. The deadline for the electronic poster was today. So I thought I’d share it with all of you. It’s meant to be an introduction to the project and highlight some of our past results. Our latest results aren’t quite ready for prime time, we’ll be able to share those results once we have them in the Fall. We’re still getting observations from Keck and other telescopes for our latest candidates and will be spending the rest of the summer working on analyzing those observations.
Cheers,
~Meg
Proof is in the Pudding
Hi,
I wanted to give a brief update on the short period planets paper that just recently got accepted to Astrophysical Journal. Once you’ve gone through the referee process and the paper gets accepted. You go through the editorial stage of the paper where the journal formats your paper into the nice two column format of the journal and puts your figures into the text so that everything looks seamless and coherent. On top of that, a copy editor reads your paper searching and correcting for typos, grammatical errors, and formatting errors. Once this has been done, you receive the proofs of your paper, what it will look like in the final print version in the general. One version, the ‘redline’ copy, highlights the corrections and changes from the copy editor and the other shows how the paper will look in journal format including where all the figures will be positioned. So I just got the proofs for my paper a few days, and I’ve gone through and checked the copy editor’s modifications. For the ones I disagree with, I can submit a response explaining my reasoning and those edits may be modified. Now that the proofs are in and reviewed, the next step is publication (expected to be formally in August). The paper is online in pre-print format so everyone can read the results early, but the publishing in the journal is considered the official stamp of approval that the publication is scientifically valid and that the results have been peer-reviewed.
So what’s next? We’ll right now I’m working on some observing follow-up of our highest priority planet candidates. We’ve been getting follow-up observations to help study and confirm if these are real planet transits. I was helping to observe on the Keck telescopes Monday and Tuesday nights (Hawaiian time). I didn’t get to be go out Hawaii or Mauna Kea. I was observing remotely from the comforts of home (well, the Yale Keck remote observing run across from my office in New Haven). So Tuesday and Wednesday morning on the East coast I was helping to drive the Keck I around and take high resolution spectra including observations of a Planet Hunters candidate or two. Additionally, we’re looking for new planet candidates and we could use your help. I’ve run an adapted version of my transit selection pipeline from Q1 data and I’ve applied it to all the classifications from Q2-Q5 that we have complete. We have a large list of potential unknown planet candidates. We need help sorting through identifying those light curves from our top list have actual planet transits similar to what we did for the short period planet analysis. If you’d like to help with the sorting, we could use all the help we can get. Go to http://www.review.planethunters.org now.
Clear Skies,
~Meg
Familiar candidate appears elsewhere…
A recent paper announcing 16 new candidates from Kepler data by Princeton astronomers Xu Huang, Gaspar Bakos and Joel Hartman has got some attention on Talk, as one of the candidates they report is the one we identified back during our mad rush through Stargazing Live. While that’s been clearly identified on talk for a while, we didn’t rush to write a paper on it, and a few people have asked why not given that the glory of first in print has now gone to the Princeton team.
The main reason is that for this candidate in particular, we wanted to wait until we could get plenty of follow-up data. Tom Barclay, in the last post on the blog talked through the various scenarios that lead to false positives, even when the transits themselves are real, and the only way to be sure you’re looking at a real planet is to go observing. We’ve been doing that for a selection of Planet Hunters candidates, including this one, and hope to report good news soon. In the meantime, for this candidate, we chose not to write up a separate paper until we had better evidence – there’s some circumstantial evidence that there might be some interference from a background source and we wanted to be careful. (We weren’t helped by the fact that the Kepler field is best seen in the summer…)
In such a new field as planet hunting, it’s not at all clear what we should make public early and what we should hang on to. This was one call it seems we got wrong, but we’re still looking forward to analysing our observations and seeing what’s really going on with this star. One thing we know we can do better is be faster to find interesting candidates, and if you want to help us with that you could do a lot worse than head over to a new part of the site where we’ve asking your help to review our best candidates.
Chris
Turning a planet candidate into a bona fide planet
Today we have a guest post by Tom Barclay, Tom is a member of the Kepler team and also a collaborator and co-author on our second Planet Hunters paper. Tom is a research scientist supporting the work of the Kepler mission. He got his Ph.D from University College London in the UK before moving to NASA Ames Research Center in California where he spends time improving the quality of the Kepler data products, finding new planet candidates and supporting the wider astrophysics community.
The Kepler team have found several thousand exoplanet candidates. The number of targets showing transit-like signals is increasing on a nearly daily basis as we search through light curves. However, these candidates are just that, candidates. Even though the planet candidates list is thought to have a high degree of fidelity, meaning that the vast majority of candidates are indeed real planets (somewhere in the region of 90%), it requires significant amounts of time and resources to turn a planet candidate into a planet.
I’ll start by being careful with my terminology. The Kepler team use two terms when deciding a candidate is a planet. Confirmation and validation. The former generally only used when we have spectroscopic radial velocity follow-up observations. These are measurements of the wobble induced on the star by the mass of the planet. The planet and star orbit a common point in space. When the planet is moving towards us the star moves away, and vice versa. When the star moves away it gets a little redder and when it moves towards us it get a little bluer. We measure these shifts and it tells us how fast the star is moving in along out line of sight.
Radial velocity measurements in combination with a transit give the planet’s mass and radius. A radial velocity detection of a planetary mass object (normally taken to be less than 13 Jupiter masses) is very unlikely to be erroneous and we are therefore happy to confirm the existence of a planet.
In order to measure a radial velocity a planet must be close enough and massive enough to have a measurable effect on the star. The best instruments currently available are sensitive to a periodic change in radial velocity of around 1 m/s and even getting this precision requires a bright star. The Earth causes a radial velocity pull on the Sun of around 10 cm/s, measuring with this precision is out of the question with currently available instruments. We therefore require another method to use another method if we want to turn small planet candidates into planet.
Validation of a planet
Validation of a planet applies when we use a statistical argument to say that it is much more likely that the transit signal is caused by a planet passing in front of the the target star (I’ll call it star A) that it is to be caused by something else.
There are 4 main ‘something else’, or false positive, scenarios we consider.
- A background eclipsing binary
- A background planetary system*
- An eclipsing binary physically associated with the star A
- A transiting star-planet system physically associated with star A (***There is some debate on whether a planet orbiting a star other than star A should really be considered a false positive. It is still a planet but it does contaminates our statistics on how many small planet are in the Galaxy.**)
A background eclipsing binary is a system of two stars that are appear fainter than star A, usually because they are far away (although they could be intrinsically faint stars which are, counter-intuitively, in the foreground between us and star A). The two fainter stars pass in front of one another much like a transiting planet does and cause a periodic dip in brightness. Because star A is much brighter than the eclipsing system, the eclipse depth appears to be much shallower than it really is and hence the eclipse looks similar to planet transiting star A.
A background planetary system is much the same as scenario (1) but the fainter system contains a star and a planet instead of two stars. If we think the transit is of a planet around the larger star A, we get the planet radius wrong. If we are not careful this scenario could cause us to claim a Jupiter-sized planet is Earth-sized.
Scenario (3) is what is known as a hierarchical triple. There are three stars in the system, star A and two lower mass stars which eclipse each other and orbit around the same center of mass as star A. This is more common than one would initially think guess. Around half of all stars are members of binary systems and in the region of 10% of these are triple or multiple star systems. The light from star A washes out the eclipse of the smaller stars and the eclipse looks much more shallow than it intrinsically is.
Finally, there is the case where a star-planet system orbits star A. The depth of the transit is decreased by the presence of extra light from star A and we get the planet radius wrong.
We try to obtain high resolution images using fancy techniques like adaptive optics imaging which changes the shape of one of the telescope’s mirrors to correct for the movement of the air in the atmosphere. These images allow us to see very close to the star and therefore look for other stars nearby in the image that could cause the transit-like signal. Typically if we don’t see star nearby star A we are able to say there are no stars further than 0.1 arcseconds away (0.00003 degrees) which could cause the transit-like signal. We are then able to make use of models of our Galaxy to predict the probability that there is a star in the right brightness range and within the allowed separation from star A that could mimic the transit signal. It is common for us to be able to say there is less than one in a million chance of a there being an allowed background star. When we take into account the probability that a background star is an eclipsing binary or hosts a planet the result is usually that it is very unlikely that there is a background eclipsing binary or star-planet system.
Ruling out a physically associated star-planet or eclipsing binary system can be much more challenging. We can again use the high resolution imaging but it is much more likely that a companion star is very close to star A than is the case for a background star. One thing on our side is that the shape of the transit can be used to rule out a stellar eclipse: eclipses are usually much more ‘V-shaped’ than the typically ‘U-shaped’ planet transit. We can often say that we cannot fit the shape we observe with a stellar binary. It is also possible to rule out planet transits around a smaller star because the timescale of the ingress and egress (the part of a transit where the planet is moving into and out of transit) does not agree with the transit depth as both these piece of information yield the planet radius. However, we really need good signal-to-noise in order to place firm constraints on the ingress and egress durations. Even so, it always gives us some information even if it is not particularly constraining and this can be used to calculate a false positive probability.
The final step is to sum up the combined false positive probabilities from the different scenarios and compare that to the probability that the transit signal is due to a planet transit around star A. If the transit scenario is much more likely (say 1000 times more likely) than a false positive we claim the planet is validated. On other occasions we have to hold our hands up and say we can’t rule out the false positive scenario with a high enough degree of confidence and the source of the signal remains a planet candidate.
The case where stars host multiple planet candidates, such as that found by the Planet Hunter in the paper by Chris Lintott, is a particularly interesting one. This is because the probability that the a multi-planet candidate system contains a false positive is much lower than for single planet candidates system, somewhere in the region of 50 times less likely. This makes validation much easier.
Planet Hunters have already shown they can find these multi-planet system. Keep searching a more will appear, especially long period ones. There is a good chance that there is an Earth-like planet hiding somewhere in the data currently available.
Closer to Home
As I write this blog post, the transit of Venus is ongoing, with Venus finishing its slow march across the face of the Sun in less than an hour. The next time this will event will come around will be well after out life times in December 2117. The internet has been abuzz with these breathtaking images of the transit from all over the world from space telescopes,ground-based telescopes, even iphones strapped to solar eclipse glasses(!). I wanted to share my impressions of the event and how that ties into what we do at Planet Hunters .
For me personally, I wasn’t expecting to see the transit of Venus from New Haven. Yale’s Leitner Family Observatory was planning events, but it was cloudy starting in the morning, and it was predicted to be that way all day. I still packed the eclipse glasses, that I had gotten from the conference in Japan that I was at a few weeks back (the conference ended the day before the annular solar eclipse), in my bag before heading out this morning. I was hoping but not holding my breath for there to be a clearing of the clouds later in the afternoon, but it had rained midday. The clouds were thinning a bit in the afternoon, teasing with some small glimpses of the Sun or a brief moment where the sunlight could be seen trying to peak through. I remember on one of my first observing runs, when the weather was bad talking to the lead observer, the older graduate student in my research group. I remember her telling me about sucker holes in clouds, holes in the otherwise thick cloud cover. They happen, but not go to chasing them with your telescope because the can close and move just as fast as they appeared. I was hoping maybe we’d get a clearing in the clouds but it didn’t look like it was going to.
I was already resided to the fact I was going to be watching online on the live streams. I had even lamented to Chris who’s in Norway, in the land of the midnight Sun, for the transit who also had clouds from horizon to horizon for the start of the event. I got on the bus to go home, and noticed what I thought was sunlight on the buildings. I got off a few stops later so I could walk the rest of the way home (or to the observatory just in case), and low and behold – a sucker hole had opened and there was the Sun struggling but nearly all the way out of the clouds staring back at me right above the astronomy building. I pulled out those eclipse glasses and my own eye glasses (that I rarely wear) and there it was. A bit of cloud still coming over in waves across the Sun’s disk, but there was a black spec on the top right. That was Venus! I made it to the Leitner Observatory where the other postdocs and grad students were, sharing our eclipse glasses to the members of the public who had come to the event and were in line to see through the solar telescopes. We also also got a chance to see the transit through solar telescopes. I captured a neat image from a solar spotter that an amateur astronomer (and also a fellow Planet Hunter) had kindly brought along. As the sun set, the clouds came back as quickly as they had parted and the sky was covered and grey again.
View of Venus and the clouds on the Sun’s disk from a sun spotter in New Haven
I have to say it was truly breathtaking and I hope you got to see it yourself and if you didn’t see it outside that you were able to view the transit online. It is amazing to think that that small dark sphere is really a planet moving in front of our Sun.
One of things for me that is so fascinating is how our view of exoplanets has changed since the last transit of Venus, which occurred in 2004. Kepler hadn’t launched, we didn’t have over 2000 transiting planet candidates (or Planet Hunters 🙂 ) Kepler really has changed how we view the universe around us, with extreme worlds orbiting two stars as well as the first detection of Earth-sized planets, and the first set of planets orbiting in the habitable zone of their stars (meaning if they were rocky or had rocky moons they might be able to have liquid water pool on their surfaces).
The same way that Venus is blocking out part of the Sun’s light (about 0.1%), is the way we identify planets in the Kepler light curves with Planet Hunters. If aliens in another solar system could watch the Sun today/yesterday, they would see a drop in light of about 0.01% for nearly 7 hours indicating Venus’s presence. We see the drops in the light curves indicative of a planet orbiting their parent stars in the Kepler field. We’ve already found four new planet candidates that weren’t previously identified by the Kepler team but there’s something different in seeing the light curve compared to seeing the Venus transit live. I’ve always known these planet candidates we’re finding are marching across the disks of their parent stars, but seeing the transit of Venus it felt real. I’m heading back to spend the rest of the night working on my next Planet Hunters paper, thinking about the transits and planetary systems we’re finding and it feels just a bit more familiar…..a little bit closer to home….
~Meg
PS. Fancy looking for some more transiting planets, come to the Planet Hunters website and give it a try.
Assessing the Kepler Inventory of Short Period Planets
You might remember that I’ve been working on a systematic search of the Q1 light curves to examine the frequencies of large planets (> 2 R⊕ -Earth radii) on orbits less than 15 days. I’m happy to announce that my paper titled “Planet Hunters: Assessing the Kepler Inventory of Short Period Planets” has just been accepted to Astrophysical Journal. The paper is available on-line here if you’d like to read it (warning: it’s quite long coming in at 22 pages of single spaced text, 13 figures, and 8 tables!), but I’ll give the highlights below.
We wanted to see for Q1 light curves, how well we could find planets and what might be left remaining there to be found compared to the known Kepler sample of planets. I think this important because Planet Hunters can serve as a separate estimate of the planet abundance and Kepler detection efficiency. I decided first to concentrate the search of planets with periods less than 15 days so that I was certain there would be at least two transits visible in the Q1 light curve. I thought it might be harder for us to identify transits if there was only one dip, so I thought it would be a good idea to start where there were at least transits.
To figure out which of the light curves had transits, I developed an algorithm to combine the multiple classifications for each light curve (for Q1 on average 10 people classified each ~33 day Kepler light curve) by developing a weighting scheme based on the majority vote. What the weights are doing is really just helping me pay a bit more attention to those that are a bit more sensitive at finding transits when combining the results from everyone who classified that light curve. The weighting scheme makes us more sensitive to transits than if I just took the majority vote for each light curve and helps to decrease the false positives. Below is the distribution of user weights for Q1 classifiers.
Distribution of Q1 Planet Hunters user weights binned in 0.5 bins
Using the user weights, I am able to give each light curve a ‘transit’ score (the sum of the user weights who marked a transit box divided by the sum of the user weights for everyone who classified the light curve). To narrow the list from 150,000 light curves, I picked those light curves that had ‘transit’ scores greater than 0.5 as my initial list of candidates. I applied several additional cuts to widdle down the list (you can read all about those details in the paper). That left about 3000 light curves and approximately 4000 simulations to go through. So to identify those that had at least two transits in them, we turned to a second round of review where light curves were presented in a separate interface and volunteers were asked whether they could see at least two transits (ignoring the depths being the same or not) in the light curve and asked to answer either asked to answer ‘yes’, ‘no’ or ‘maybe’ to the question. Those light curves where the majority of classifiers said ‘yes’ were moved on to review by the science team. A big thank you to everyone helped out with the Round 2 review; your efforts are acknowledged here. As always we acknowledge all those who contribute to Planet Hunters science on our authors page.
At the end of the search after removing all the known planet candidates and transit false positives known before February 2012, there were 7 light curves that have transit-like events but not on the original Kepler candidates list published back in 2011 that used only Quarters 1 and 2. I show example transits from each of these 7 light curves in the Figure below. One of these light curves turns out to be one of the candidates from our first paper and another one was part of our co-discoveries with the Kepler team. Even those these 7 light curves weren’t found in the first Kepler candidate releases, they now have been found in the latest iteration of the Kepler candidate list released earlier this year, where they’ve used an updated and improved versions of their detection and data validation pipelines. So what that shows is that the Kepler detection and validation processes has indeed gotten better, but there’s more that we can say.
Zoom-in of selected transits for each set of transit identified visible in short period candi- date light curves remaining after Round 2 review and visual inspection. Visually the science team could identify two separate sets of repeating transits in the mutli-planet KIC 8240797, 9729691, and 11551692 based on the user drawn boxes We note that the snapshot of KIC 8240797 contains two independent transit events.
Now that we know what new things we found, and that there wasn’t anything more than the 7 candidates that are now KOIs on the latest Kepler candidate list, we can look at what that says for the completeness of the short period planet inventory. Using the simulations that you’ve helped classify, I was able to look at how good Planet Hunters is at detecting planets of different sizes on orbits less than 15 days. I randomly selected about 7000 light curves that at the time weren’t known to have transiting planets or were not eclipsing binaries and inject synthetic transits into them for varying planet radii (ranging from 2- 15 R⊕) and periods less than 15 days. The simulations are really important because one completed I could see what which of the simulations made it to the end of my candiate pipeline and which ones didn’t. Having the results from those classifications really made the heart of the paper, because we could show independent of the Kepler planet candidates and detection and validation processes, what we were sensitive to.
Efficiency recovery rate for simulated planet transits with orbital periods between 0.5 and 15 days and radii between 2 and 15 R⊕.
What was striking to me, was our detection efficiency is basically independent of orbital period and that whether there were 2 or 15 transits in the light curve, they were just as easily identified. I think this bodes well for us being just as sensitive to single transit events (I’m starting to work on testing that now). Although performance drops rapidly for smaller radii, ≥ 4 R⊕ Planet Hunters is ≥ 85% efficient at identifying transit signals for planets with periods less than 15 days for the Kepler sample of target stars. For 2-3 R⊕ planets, the recovery rate for < 15 day orbits drops to 40%. I compared to the Kepler planet candidates and found similar results (which is a good check).
Our high recovery rate of both ≥4 R⊕ simulations and Kepler planet candidates and the lack of additional candidates not recovered by the improved Kepler detection and data validation routines and procedures suggests the Kepler inventory of ≥4 R⊕ short period planets is nearly complete!
The Transit of Venus and the Scale of the Universe
I’ll start by introducing myself as I’m not involved in Planethunters, but Meg asked me if I could write an article for you here about the Transit of Venus after I mentioned on Twitter that I was enjoying researching the topic for a talk I’m giving. I’m a Research Fellow at the Institute of Cosmology and Gravitation at the University of Portsmouth/SEPNet (South East Physics Network), and I’m funded by The Leverhulme Trust as an Early Career Fellow to work on Galaxy Zoo science. I’ve been part of the Galaxy Zoo science team since 2008 and I lead the studies of the interesting class of red spirals which were found by Galaxy Zoo, and am also interested in the role bars seem to have in slowing down star formation in spiral galaxies. You can read blog posts I’ve done for the Galaxy Zoo blog, which include explanations of these studies at http://blog.galaxyzoo.org/author/karenlmasters/
On 5th/6th June 2012 Venus will pass between the Earth and the Sun. It’s shadow will cross the Sun taking about 6 hours in total, although the length of that which is visible varies significantly depending on where you live on Earth. In the USA the beginning of the transit will be visible as the Sun begins to set on 5th June, while in the UK we’ll see the end of the transit after the Sun rises on 6th June.
The planet Venus orbits the Sun inside the orbit of the Earth, and passes between the Earth and the Sun quite frequently. However the planes of the two orbits aren’t quite aligned, so most of the time Venus passes either above or below the disk of the Sun. Actual transits are rare, but very predictable. They happen in pairs separated by 8 years, and then after each pair follows a period of either 121.5 or 105.5 years without any transits. The 2012 transit is the second of a pair – astronomers around the world viewed the first transit of the modern age in 2004, and the next transit won’t happen until December 2117.
To view a transit of Venus does not need to be technically challenging, but in its simplest form involves looking at the Sun – so some safety precautions must be taken. You must never look directly at the Sun! Serious eye damage or even blindness could occur if you did that, so take this warning seriously! My advice for viewing the transit if you are not a keen amateur astronomer, or able to get to a viewing party, is to either get your hands on a pair of Safe Solar Viewers (like these ones from Astronomers Without Borders), or to view the event via a web cam (like this one from Hawaii where the entire transit is visible, or GLORIA which is planning to show feeds from Australia, Japan and Norway). This last method also has the advantage (especially for UK viewers) of not being affected by local cloud cover, and has exactly zero risk of eye damage! If you come to one of my talks I hope to have solar viewers to hand out.
So why should you care about a black dot crossing the Sun, or perhaps more pertinently, why do I, and an astronomer who studies galaxies, and works in a cosmology department care enough to write a blog post about it and give several talks? Well historically transits of Venus have been very important in helping astronomers understand the scale of the solar system, and from that the scale of the Universe. Basically when Venus crosses the Sun we know that it, the Sun and the Earth are all in a straight line. Very slight differences in the viewing angle from two observers on the Earth can then be used along with our basic knowledge of trigometry to measure the distance to the Sun. For over 100 years, the distance to the Sun measured this way was the most accurate measurement we had.
From knowing the distance to the Sun, we can use slight changes in the apparent position of nearby stars as the Earth orbits the Sun to get their distances (more triangles – this is called the parallax method), and from those stars we calibrate methods which use stars of known or estimated brightness to estimate distances to nearby galaxies, and we jump from distances to nearby galaxies to more distant galaxies and eventually the whole universe. The distances to faraway galaxies have taught us that the universe is expanding and started in a Big Bang around 15 billion years ago, and even if we go to the observations that suggest the universe contains a mysterious “Dark energy” (which won the 2011 Nobel Prize in Physics), they are ultimately based on us knowing the distance to the Sun. So that’s why I think it’s important.
Here in Portsmouth we’re running a workshop about the transit of Venus on HMS Warrior, in the Portsmouth Historic Dockyard. We plan to demonstrate the triangulation method by using it to estimate the distance from the Warrior to the local landmark “The Spinnaker Tower”. Two people will stand on the desk and we’ll mark out from them the direction to the Spinnaker Tower. Making one of the angles a right angle, we can then estimate the distance to Spinnaker (which is about 300m) by d = b/cos A, where A is the other angle, and b is the distance between the two people on deck.
Google Map image of Portsmouth including HMS Warrior (top) and The Spinnaker Tower (bottom).
This isn’t exactly the method used in the historical measurements, but it demonstrates the idea. Of course when observing the transit of Venus from two widely separated places on Earth, it’s not exactly easy to measure the angle between the sight lines. What Edmund Halley figured out in 1678 was that if you could measure the times when Venus stars and ends its transit you can get at the same information.
In all of recorded history, we have records of a total of six transits of Venus that have been observed (1639, 1761, 1769, 1874, 1882 and 2004). You have to feel sorry for Johannes Kepler, who predicted the transit which occured in 1631, but then died in 1630. No-one is recorded to have used his prediction that year. Jerimiah Horrocks and Willam Crabtree (two British astronomers who were friends) have the honour of being the first humans known to have seen Venus transit. Horrocks found and improved Kepler’s earlier predictions, and both men successfully observed the 1639 transit from Northern England (in December!).
Scientific observation of the transit started in the 18th century following Halley’s suggestion to use it to measure the distance to the Sun. There is a hugely rich and entertaining history of these expeditions to view the transit, and several books have been published. I’m just going to tell you a couple of the stories which struck my interest! A lot more, and links to some of the books can be found via the Wikipedia page on Venus Transits, TransitofVenus.org, and TransitofVenus.nl.
For the 1761 transit, two famous explorers got involved. Mason and Dixon (still in Britain, and before they set off to map the USA) were commissioned by the Royal Society to observe the transit from Sumatra. They met in Portsmouth, and set sail from here on HMS Seahorse (which a decade later would have a famous midshipman named Horation Nelson). Enroute to Sumatra they got attacked by the French (the French and English being at war then), and decided to give up. They wrote the Royal Society of their intentions and were promptly told they better get right back on the ship to observe the transit or else. They did, and eventually ended up viewing if from South Africa.
The 1769 transit also had some famous viewers. Captain Cook was ordered to set sail in HMS Endeavour, partially to observe the transit from Tahiti, but then to continue on and look for the mythical “Australia”. On that trip they didn’t find Australia, but they did land and claim New Zealand. And in Tahiti, they set up a “Fort Venus” from which to stay safe from the natives and view the transit. This voyage is being repeated this year in a replica vessel, and you can follow along at the HMB Endeavour Website.
These 18th century observations results in a measurement of the distance to the Sun of 153+/-1 million kilometers, which was a huge improvement over previous estimates, but not as good as they expected. The timings were thwarted by something which came known as the “black drop effect” where the shadow of Venus seems to bleed into the edge of the Sun. This meant that the start time of the transit could not be measured to better than a few seconds.
The black drop effect, photographed in 2004.
In 1874 several more expeditions set out, including several on Royal Navy Ships, such as HMS Volage, one of the largest mixed sail and steam ships ever built, which ferries the British expedition to the Kerguelen Island in the Southern Indian Ocean. Such measurements helped improve the distance measurement to 149.59+/-0.3 million kilometers. By 1881 it had been decided the distance to the Sun could be estimated better by other methods, although several expeditions still set out, and the first photographs of the transit were taken.
A photograph of the 1882 transit, taken by a US expedition.
The current best distance to the Sun is 149.5978707 million kilometers, +/- 3 metres, measured using radar ranging to the inner planets. It’s known so accurately that we can measure it’s changing, growing about 15 metres every century.
The first transit of Venus to happen in the modern age was in 2004. You can find videos of this event (like the one below), which I was lucky to view from a small observatory near Ithaca, NY while I was studying for my PhD in Astronomy at Cornell University.
For the 2012 transit, apart from encouraging people to view the event as a last in your life time chance, there are couple of new developments. First smart phone technology which didn’t even exist in 2004 has allowed the development of a “Transit of Venus Ap“. In this Ap you can input the time you view the transit starting and/or ending, and participate in a global experiment to measure the distance to the Sun. Download the Ap in advance to practice inputing your measurement.
Interest in exoplanets has also significantly grown, including the signature that the atmospheres of those planets might have in the observed spectrum of a star when the planet is transiting. The Hubble Space Telescope will try to simulate this type of observation during the transit of Venus, observing the light from the Sun reflected by the Moon (if HST looked at the Sun it would be destroyed) to search for the signature of the atmosphere of Venus. You can read more about the plans on the NASA website.
Anyway I encourage you to get out there, or get online and view the last transit in our lifetime. Use the resources attransitofvenus.org, to work out the timings of the transit from your location, or search for local events. Other useful resources are the Royal Astronomical Society page on the Transit of Venus, in the UK, the HM Nautical Almanac Office. Also of possible interest, the Royal Observatory, Greenwich has a Venus Transit page, and a special (free) exhibit running until September 2012. And of course there’s a special Planethunters page on the transit too.
Transit of Venus: Live
In June 2012 people all over the world will watch the planet Venus transit across the Sun. Planet Hunters is all about spotting planets as they move across the face of a star so we thought it would be good to share the event with everyone. Venus will pass directly between the Earth and Sun on the night of June 5th and the morning of June 6th. This historic event can be seen from many parts of the world and will not happen again for 105 years!
As the map above shows, most people will only see part of the transit. With the help of the GLORIA team, we’ll be showing a live feed of the whole event on the Planet Hunters site. The webcast is being streamed from Tromsø, Sapporo and Cairns and will feature commentary in English and Spanish during the key parts of the event.
Check out our guide to the Transit of Venus, which we’ll update as we approach the event itself. It covers a basic history of the transits, and include information on when and where to see it. It also links to other useful resources for the event, including a Transit Guide from the GLORIA group, and the NASA observers handbook links. We hope you’ll try to see the transit when it happens, but if you’re unable to for some reason, then the webcast means that you can still be a part of this last-chance astronomical event.
Awesome People: More from ZooCon1
Today we have a guest post by Jules, fellow Planet Hunter and zooite who attended the ZooCon1. Jules is a lead moderator and blogger for the Solar Stormwatch and Moon Zoo forums as well as a volunteer on the Zooniverse Advisory Board.
Just back from the very first #zoocon1 in Chicago. I attended as a volunteer on the Zooniverse Advisory Board. As Meg said it was a chance for the science teams from new projects to meet with and learn from representatives of current projects and for everybody to meet up with Zooniverse techies and developers. It made sense then for some of the “old hands” to present an overview of their own projects. Meg’s Planet Hunters talk was particularly interesting as it highlighted the value of Talk and the great collaborative work being done there by volunteers.
A brief foray into data reduction showed the kind of work necessary to make the clicks usable. For example, there are 5,508 stars with possible transits. Removing all pulsating stars, which can be mistaken for transits, reduced the number of candidates to 3,404. Further examination of these transits reduced the pool further to 77 transit candidates – a much more manageable number.
Here’s Meg in action demonstrating the light curves of different sized planets.
The discoveries Meg highlighted included a slide showing 4 planet candidates missed by Kepler one of which is being re-investigated because of the work done by Planet Hunters. Kepler 16, the circumbinary system, also got a mention as did the impressive volunteer-led analysis on cataclysmic variables and heartbeat stars.
Old Weather, Mergers and the Milky Way Project were also put in the spotlight. Afterwards someone from one of the new projects told me how amazed they were that volunteers would want to do more than just click and another told me that they found the Planet Hunters story particularly inspiring and wanted to know how Planet Hunters had attracted these “awesome people.”
Well that’s Citizen Science for you. Volunteers come with a great mix of interests, skills and the knack of finding treasure!
Planet Hunters 