Duotransits

In this project, my collaborators and I laid out a foundation to find long-period planets with the K2 and TESS space missions. The paper, accepted to the Astronomical Journal, outlines the new method and its use on six planet candidate systems, including a 4-planet system.


In 2018, I watched the TESS spacecraft launch from Cape Canaveral on the big screen in Campbell Hall in Berkeley knowing I would soon have the opportunity to use its data to find and study exoplanets. TESS had a mission goal of finding earth-like planets around the habitable zones of cooler stars. As a result, it was designed to observe large regions of the sky for 27 days at a stretch before moving on, finding planets with an orbital period of 10 days at maximum. Such planets, orbiting their host stars much closer than the orbit of Mercury to the Sun, would nevertheless be in the habitable zone of a cool star.

But what about planets with longer orbital periods, more similar to the 365 days of the Earth? The Kepler Space Telescope looked for such planets before the failure of its reaction wheels in 2012, which cut short its 5-year stare on one location of the sky. The rejuvanated K2 mission used a similar strategy to the TESS mission, staring at one location for about 85 days before moving on.

However, there was one way to detect longer period planets–by utilizing the few locations where K2 re-observed a location after a year or two. We realized these observations, namely in Campaigns 5, 16 and 18 in K2, could be used to look for long period planets with transits in each baseline. This also would make these campaigns a perfect dress rehearsal for finding long-period planets with TESS.

The challenge with finding these long-period planets is that we only see one transit in each observation, and we may have missed any number of transits in between. This means we only know the longest possible period, but the true period could be any integer division of the longest period–a problem reminiscent of aliasing in signal processing. While it’s not possible to find the true period with just the K2 or TESS observations, we can place constraints on the period and help to pin down the true period with ground-based observations.

It became apparent to us that when we see these “duotransits,” not ever period is equally likely. For instance, given two observations of a set duration spanned by a long gap, a shorter period planet would be more likely to transit in each observation, and a longer period planet more easily missed. This means if we see one of these duotransits, even without looking at the data, we can say it is more likely to have a shorter period.

In addition, we can also use the transit shape to help us pin down the period. Longer period planets will have longer duration transits. This, unfortunately, is somewhat degenerate with the impact parameter and eccentricity, which also influence the duration of the transit. Nevertheless, we can still use the transit shape to make good constraints on the period.

But how do we take into account all this information quantitatively? Our main idea was to set up a Bayesian framework to fit these duo transit observations. This means we need a prior that weighs the periods by how likely they are to cause the observed duotransit. Then, we can perform a fit that takes into account the evidence, which in this case is the shape of the transit, and arrive at a posterior, which is the distribution showing how likely each period alias is to be the true period of the planet.

We came up with the first general period prior that applies to any number of transits seen (or not seen) in any number of baselines. This is exciting because it will be applicable in cases where we see perhaps one transit in one campaign/sector and then no transit in the other–or in the case of our K2 observations, three sets of observations in which there may or may not be transits.

Using the list of single-transits found in Campaign 5 of K2 by citizen scientists, we looked through Campaigns 16 and 18 to search for further transits. We ended up finding 6 planet systems where we also see transits in either Campaign 16 or 18 or both.

Applying our method to these systems, we can construct a distribution of possible periods for each planet. That result is shown below, including a 4-planet system for which we characterized the system’s architecture for the first time and found period distributions for each planet.

The great thing about these new candidates is that they are both bright and long-period, filling in a niche left empty by the Kepler mission, which mainly studied faint stars that are hard to follow up from the ground, and K2 and TESS, which are designed to primarily find short period exoplanets. This can be seen in the plot below showing our new candidates against all currently discovered planet candidates:

It is my hope that this method will be useful in a few years when TESS finishes its initial pointings and starts re-observing locations on the sky. Perhaps this method could even assist in finding small planets near the habitable zones of Sun-like stars!

About Me

I’m Shashank! I’m a first year PhD student in astrophysics at University of Queensland in Brisbane, Australia. I also enjoy photography and play badminton.

Wrong guy? You might be looking for him


My life and education

2022

University of Queensland

Education

I moved to Brisbane to start a PhD at University of Queensland

2021

Berkeley

Education

I graduated from UC Berkeley with a major in astrophysics and a minor in K-12 STEM education. In addition to my studies at Berkeley, I also:

  • created and ran a 2-unit DeCal course called Beginner’s Guide to the Universe for 4 semesters.
  • played competitively for the Cal Badminton team, organized intercollegiate tournaments and served as their photographer for 3 semesters.
2017

Wilcox High School

Education

I graduated from Adrian Wilcox High School in Santa Clara, California. At Wilcox, I

  • started playing badminton and joined Wilcox’s badminton team, where I played doubles with my brother
  • created and ran the Science Club for 3 years
  • did science fair projects on astronomy and competed at the state and international level
2015

NASA

Internship

I interned at NASA AMES Research Center at the K2 Guest Observer’s office after my (and my brother’s) 10th grade science fair topic resulted in the discovery of a candidate exoplanet. While there, I worked on

  • validating, following up and assessing cluster membership of a substellar object orbiting an A-star
  • searching for exoplanets orbiting stars in open clusters using data from K2

Blog

Here’s where I post what I’ve been up to lately. These are sometimes but not always research updates. Other things I post about include things I’ve been working on for fun, personal project interests, musings and other things I think might be helpful to share.

Contents:
Apr 9th, 2023 | What is interferometry?
Jan 8th, 2022 | A 3d-printed moon nightlight
Dec 28th, 2021 | A foldable model of JWST
Dec 27th, 2021 | Hello world

What is interferometry?

For a long time I’ve been meaning to use this blog as a way of explaining my ongoing research interests to my friends who may or may not be astronomers. As I’m coming up on my first-year PhD confirmation, I’m reflecting on everything I’ve learned in the past year. As a way of procrastinating writing my confirmation report, I thought I’d sit down and write up an explanation of the technique I have been learning for much of my PhD so far: interferometry. Perhaps the visuals I’ve made may come in handy for my confirmation too.

A 3d-printed moon nightlight

I’ve been continuing to put my 3d printer through its paces recently, and found out about an intruiging use for 3d printers: printing lithophanes. Lithophanes are objects made from translucent material where the thickness is modulated to produce an image when backlit. One of the most interesting such prints I found was a spherical moon lithophane–which begged me to design and make a nightlamp surrounding it.

A foldable model of JWST

I’ve been very excited for the launch of the James Webb Space Telescope (JWST) for a long time. So long, in fact, that I’ve had a JWST poster up in my bedroom since about 2012 or 2013, when I was 12 years old.

Hello world

Hello everyone! This is the intro post for my blog, which I intend to use for semi regular research updates, musings, and side projects which I always seem to be working on. For my main research projects, check out the research porfolio on my home page.

Contact

You can email me at:

Or, you can find me on Twitter at AstroShashank

Research

My research broadly focuses on exoplanets and their host stars. I like to use techniques such as photometry, spectroscopy and polarimetry in new ways to characterize exoplanet systems in detail. Currently I am interested in hot stars with exoplanets, which exhibit a variety of effects such as gravity darkening, oblateness and pulsations, which can all be used to learn more about exoplanets around these stars.