Introducing HelloExoWorld: The quest to discover exoplanets with Warp10 and Tensorflow
update 2019: this is a repost on my own blog. original article can be read on medium.
My passion for programming was kind of late, I typed my first line of code at my engineering school. It then became a passion, something I’m willing to do at work, on my free-time, at night or the week-end. But before discovering C and other languages, I had another passion: astronomy. Every summer, I was participating at the Nuit des Etoiles, a global french event organized by numerous clubs of astronomers offering several hundreds (between 300 and 500 depending on the year) of free animation sites for the general public.
As you can see below, I was kind of young at the time!
But the sad truth is that I didn’t do any astronomy during my studies. But now, I want to get back to it and look at the sky again. There were two obstacles:
- The price of equipments
- The local weather
I was looking for something that would unit my two passions: computer and astronomy. So I started googling:
I found a lot of amazing projects using Raspberry pis, but I didn’t find something that would motivate me over the time. So I started typing over keywords, more work-related, such as time series or analytics. I found many papers related to astrophysics, but there was two keywords that were coming back: exoplanet detection.
What is an exoplanet and how to detect it?
Let’s quote our good old friend Wikipedia:
An exoplanet or extrasolar planet is a planet outside of our solar system that orbits a star.
do you know how many exoplanets that have been discovered? 3,529 confirmed planets as of 10/09/2017. I was amazed by the number of them. I started digging into the detection methods. Turns out there is one method heavily used, called the transit method. It’s like a eclipse: when the exoplanet is passing in front of the star, the photometry is varying during the transit, as shown below:
animation illustrating how a dip in the observed brightness of a star may indicate the presence of an exoplanet. Credits: NASA’s Goddard Space Flight Center
To recap, exoplanet detection using the transit method are in reality a time series analysis problem. As I’m starting to be familiar with that type of analytics thanks to my current work at OVH in Metrics Data Platform, I wanted to give it a try.
Image Credit: NASA Ames/W. Stenzel
Kepler is a space observatory launched by NASA in March 2009 to discover Earth-sized planets orbiting other stars. The loss of a second of the four reaction wheels during May 2013 put an end to the original mission. Fortunately, scientists decided to create an entirely community-driven mission called K2, to reuse the Kepler spacecraft and its assets. But furthermore, the community is also encouraged to exploit the mission’s unique open data archive. Every image taken by the satellite can be downloaded and analyzed by anyone.
More information about the telescope itself can be found here.
Where I’m going
The goal of my project is to see if I can contribute to the exoplanets search using new tools such as Warp10 and TensorFlow. Using Deep Learning to search for anomalies could be much more effective than writing WarpScript, because it is the neural network's job to learn by itself how to detect the exoplanets.
As I’m currently following Andrew Ng courses about Deep Learning, it is also a great opportunity for me to play with Tensorflow in a personal project. The project can be divided into several steps:
- Import the data
- Analyze the data using WarpScript
- Build a neural network to search for exoplanets
Let's see how the import was done!
Importing Kepler and K2 dataset
Step 0: Find the data
As mentioned previously, data are available from The Mikulski Archive for Space Telescopes or MAST. It’s a NASA funded project to support and provide the astronomical community with a variety of astronomical data archives. Both Kepler and K2 dataset are available through campaigns. Each campaign has a collection of tar files, which are containing the FITS files associated. A FITS file is an open format for images which is also containing scientific data.
FITS file representation. Image Credit: KEPLER & K2 Science Center
Step 1: ETL (Extract, Transform and Load) into Warp10
To speed-up acquisition, I developed kepler-lens to automatically download Kepler/K2 datasets and extract the needed time series into a CSV format. Kepler-lens is using two awesome libraries:
- pyKe to export the data from the FITS files to CSV (#PR69 and #PR76 have been merged).
- kplr is used to tag the dataset. With it, I can easily find stars with confirmed exoplanets or candidates.
Then Kepler2Warp10 is used to push the CSV files generated by kepler-lens to Warp10.
To ease importation, an Ansible role has been made, to spread the work across multiples small virtual machines.
Principles of transaction-oriented database recovery
The import of @NASAKepler dataset has been spread on 16 machines, just because I can 😎 pic.twitter.com/qa49tAgdzz— Pierre Zemb (@PierreZ) September 15, 2017
- 550k distincts stars
- around 50k datapoints per star
That's around 27,5 billions of measures (300GB of LevelDB files), imported on a standalone instance. The Warp10 instance is self-hosted on a dedicated Kimsufi server at OVH. Here’s the full specifications for the curious ones:
Now that the data are available, we are ready to dive into the dataset and look for exoplanets! Let's use WarpScript
!### Let's see a transit using WarpScript
For those who don’t know WarpScript, I recommend reading my previous blogpost “Engage maximum warp speed in time series analysis with WarpScript”.
Let’s first plot the data! We are going to take a well-known star called Kepler-11. It has (at least) 6 confirmed exoplanets. Let's write our first WarpScript:
The FETCH function retrieves raw datapoints from Warp10. Let’s plot the result of our script:
Mmmmh, the straight lines are representing empties period with no datapoints; they correspond to different observations. Let's divide the data and generate one time series per observation using TIMESPLIT:
To ease the display, 0 GET is used to get only the first observation. Let's see the result:
Much better. Do you see the dropouts? Those are transiting exoplanets! Now we’ll need to write a WarpScript to automatically detect transits. But that was enough for today, so we’ll cover this **in the next blogpost!**Thank you for reading! Feel free to comment and to subscribe to the twitter account!