Transit spectroscopy in action

Andrew Williams reveals how spectroscopy technologies help scientists to deepen understanding of exoplanets

Over the past two decades, astronomers have used transit spectroscopy to help deepen our understanding of planets beyond our solar system. So, what exactly is transit spectroscopy? What types of project are currently underway? And what type of equipment is used to capture the transit spectra of exoplanets?

Transit spectroscopy and Earth’s ‘fingerprint’

Transit spectroscopy comes in three main varieties. Emission spectroscopy, where the planet passes behind the star and is eclipsed from our point of view, meaning you can measure the stars light alone during eclipse and subtract that from the out of eclipse measurement to get the light being directly emitted from the planet. Phase curves, where scientists measure the entire orbit of the planet around its star and observe the different phases of the planet during its orbit. And transmission spectroscopy, where a planet passes in front of a star and the starlight is filtered through the limb of the planets’ atmosphere before being measured with telescopes. The spectrum of the star is then compared to the spectrum of the same star when a planet passes directly in front of it – a method known as ‘transiting’ a star.

This latter technique was recently used by a research team at McGill University in Canada to assemble a ‘fingerprint’ for Earth that could be used to identify a planet beyond our Solar System capable of supporting life. As Nicolas Cowan, Associate Professor and Canada Research Chair in Planetary Climate at the McGill Space Institute, McGill University, observes, although transmission spectroscopy is useful for detecting planets in the first place, the ‘truly interesting’ thing is that some starlight passes through the upper atmosphere of the planet, where it is filtered by the molecules present there, meaning observers can essentially obtain an absorption spectrum of the planet’s upper atmosphere.

“This is currently our most successful technique for assessing the composition of exoplanet atmospheres,” he says.

Transit spectroscopy and host stars like TRAPPIST-1

This method was used by Cowan’s colleague Evelyn MacDonald – formerly of McGill University, but currently a graduate student at the University of Toronto – who used solar occultation measurements from an Earth-observing satellite to generate a mock transit spectrum of Earth – essentially what aliens would see if they were to obtain a transit spectrum of Earth.

“This was the first empirical transit spectrum of the Earth at infrared wavelengths. The infrared is important because this is where molecules absorb the best, and it is precisely these wavelengths that will be monitored by the James Webb Space Telescope (JWST), due to launch in 2021,” says Cowan.

As MacDonald explains, the light blocked during transit is mostly blocked by the planet, but a small fraction is blocked by its atmosphere, and this fraction changes with wavelength because each molecule in the atmosphere absorbs more strongly at some wavelengths than others. For an exoplanet, we would observe this as the planet appearing larger at wavelengths where more absorption is happening. The differences are small and difficult to detect, but can in principle tell us which molecules are in the planet’s atmosphere. The McGill team found that for a planet with Earth’s atmospheric composition but a smaller host star like TRAPPIST-1, carbon dioxide is ‘by far the easiest molecule to detect with JWST.’

“Molecules like water vapour, which could suggest habitability, or ozone and methane, which could be signs of life, are more difficult to detect because the spectral features they produce are much smaller in amplitude. Such a planet could be observed with JWST for a few transits to detect carbon dioxide, which would be an early indication that the planet has an atmosphere,” says MacDonald.

“It would then take many more transits to detect these other molecules. A planet with the same molecules but different concentrations would have spectral features of different sizes, so the amount of observing time required to detect these molecules could vary,” she adds.

Fourier transform spectrometer data developing Earth’s ‘fingerprint’

As part of her work on developing an Earth ‘fingerprint’, MacDonald used data from the Atmospheric Chemistry Experiment’s Fourier Transform Spectrometer (ACE-FTS) onboard the SCISAT satellite. The ACE-FTS, designed and built by Québec City-based ABB Bomem, measures the fraction of sunlight that passes through different regions of Earth’s atmosphere at sunrise or sunset.

“The data is separated by latitude, season and altitude. Since different regions of the atmosphere cannot be resolved in exoplanet data, we reconstructed Earth’s transit spectrum by adding up the light blocked by each region, at each of the given altitudes, to get the total amount of light blocked by the entire atmosphere at a range of wavelengths. This is what the data would look like if Earth were observed as an exoplanet, from much farther away,” says MacDonald.

“Transit spectroscopy works well for small, cool host stars, because the planet blocks a larger portion of the star. These stars are very abundant, and many have already been found to host Earth-sized planets in the habitable zone; these planets are very close to their stars because the stars are dim. This means that they spend more time in transit than planets on wider orbits, so transit spectroscopy is the best way to characterise their atmospheres,” she adds.

Moving forward, Cowan reveals that, in the JWST era, he and his colleagues will perform transit spectroscopy on temperate terrestrial planets like those in the TRAPPIST-1 system. Goals include figuring out whether these planets have atmospheres at all and whether their atmospheres are ‘Earth-like’ - including the presence of CO2 and H2O greenhouse gases – as well as the commencement of the search for signs of extraterrestrial life, including ‘biologically produced gases such as methane and oxygen, or its spectroscopically active cousin, ozone.’

Hubble instruments improving exoplanet transit measurements

Elsewhere, Dr Hannah Wakeford, Lecturer in Astrophysics at the University of Bristol, also uses spectroscopy to deepen understanding about the nature of exoplanets by measuring and understanding what their atmospheres are like, what they are made of, how much of each molecule or atom is there, and ‘how is it moving around, being mixed or responding to the environment.’

“Many of the results thus far have shown us that planets are incredibly diverse. Not just in their sizes – for example we have discovered that there is a class of planets that are in-between that of the Earth and Neptune that are the most abundant in the galaxy – but their atmospheres are also all different in small and sometimes big ways,” she says.

One interesting planet studied by Wakeford and her team is HD 189733b, where it is likely that clouds containing magnesium-silicates such as enstatite or forsterite, more commonly referred to as sand, are caught up in 5,400mph winds – a phenomenon she describes as ‘a nightmare for any wayward traveller.’ The Bristol team have also measured the transits of planets d, e, f, and g of the tightly packed TRAPPIST-1 system and are now ‘working on analysing them to try and understand more about it.’

The majority of Wakeford’s work has been carried out using instruments on board the Hubble Space Telescope - including the G141 grism on Wide Field Camera 3 (WFC3), which was installed on Hubble in 2009 as part of Service Mission 4. WFC3 has two channels a UV channel and an IR channel. The G141 grism (grating + prism) is on the IR channel and produces a spectrum that goes from 1.1-1.7 microns. The IR channel uses a Teledyne HgCdTe detector.

To measure an exoplanet transit in front of its star, Wakeford explains that she needs to measure the star before the planet transits for typically around an hour – before measuring the transit, which can often take a few hours, then obtaining measurements of the star on its own again after the transit. This results in a continuous observation of one target of five or more hours.

“With the Hubble Space Telescope to improve efficiency the telescope is physically moved (nodded) over the course of a single exposure to spread the light out over the detector in one direction. This method of scanning allows you to collect ten times as many photons as a normal ‘stare’ exposure and also avoids entering the detectors’ non-linearity regime,” she says.

For Wakeford, the main advantages of transit spectroscopy are the ‘wide range of wavelengths that can be measured and therefore the wide range of pressures in the atmosphere probed, and molecules and atoms that can be measured in absorption or emission.’

“Unlike other techniques, with transit spectroscopy we can measure this huge range of molecules and get an understanding of where in the atmosphere it is and what local environment it is in. Each new observation teaches us something different and we then continually update our models to interpret and understand it and then predict what we need to look for nenext. It is very exciting,” she adds.

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