Entrevista com Heather Knutson

Heather Knutson nasceu na Califórnia mas passou grande parte da infância numa base militar no atol de Kwajalein, nas Ilhas Marshall, no Oceano Pacífico. Durante a juventude regressou ao continente onde prosseguiu os seus estudos culminando com uma licenciatura em Física, com distinção, pela Universidade de Johns Hopkins em Baltimore, Maryland. Para o seu doutoramento, Knutson dedicou-se ao estudo e caracterização das atmosferas dos exoplanetas e teve como supervisor o professor David Charbonneau, da Universidade de Harvard. Depois de obter o grau, em Janeiro de 2009, e de uma experiência breve como “post-doc”, Knutson aceitou o lugar de Miller Fellow no Departamento de Astronomia da Universidade da Califórnia em Berkeley, cargo que ocupa na actualidade.

Apesar da sua juventude Heather Knutson é já uma das figuras de topo na área dos exoplanetas. Heather aceitou amavelmente dar uma entrevista ao AstroPT que passamos a transcrever.

[AstroPT] – You spent part of your childhood in the island of Kwajalein, a coral atoll in the middle of the Pacific ocean. Did you become interested in astronomy there ? How does a child experience the night sky in such a remote, idyllic place ?

[H. Knutson] – It was completely, utterly dark at night, so of course the sky was amazing. I did a little bit of stargazing when I was growing up, but I remember the one time I mentioned the idea of becoming an astronomer to my parents (I was probably 7 or 8 at the time), they managed to convince me that no one ever actually made a living doing astronomy. Since the island I lived on was a U.S. army base dedicated to the missile defense program, I had lots of very practical role models (mainly engineers and physicists), so it was probably a foregone conclusion when I decided to major in physics in college. Halfway through college I decided to do a summer internship at the Space Telescope Science Institute (home of the famous Hubble Space Telescope!), and it was that internship that finally convinced me to pursue a career in astronomy.

(Vista aérea do atol de Kwajalein onde Heather Knutson passou parte da infância. Crédito: Kwajalein Range Services.)

[AstroPT] – What is your scientific background ? What are your main research interests and what projects are you involved in ?

[H. Knutson] – I received my undergraduate degree in physics from Johns Hopkins University in 2004, and my Ph.D. in astronomy from Harvard University in 2009. I did my thesis work with Prof. David Charbonneau, on “Portraits of Distant Worlds: Characterizing the Properties of Extrasolar Planets”. As you can probably tell from my thesis, my primary interest is in studying the properties of planets orbiting stars other than the sun, which we call extrasolar planets. Some big-picture questions include:

  • How common are planets around other stars?
  • What kinds of planets do we find? Gas giants like Jupiter? Smaller, rockier planets like Earth?
  • Do these planets have atmospheres, and if so, what are they made of?
  • What is the weather like on these worlds?

Right now my focus is primarily on the last two questions (there are other groups who are doing wonderful work on the first two areas). I study the properties of exoplanets using observations of transiting systems, where the planet periodically passes in front of and then behind its host star. The nice thing about this technique is that it allows us to study planets that are too far and too faint to image directly (i.e., take a picture where you see the planet separate from its host star, and not as a single blurry point of light).

[AstroPT] – You have done extensive research on transiting exoplanets, namely Hot Jupiters. Besides the possibility of determining the inclination of the planetary orbit and with it the planetary radius and ultimately its density and bulk composition, what other kind of information can we gather on these planets by observing their transits ?

[H. Knutson] – By measuring the decrease in light as the planet passes in front of its host star, we can measure the planet’s radius and the duration of the eclipse tells us the path it takes across the face of the star, which corresponds to its orbital inclination. However, if the planet in question has an atmosphere, that atmosphere will be opaque at some wavelength and transparent at others, resulting in deeper or shallower transits. By measuring the wavelength-dependence of the transit depth, we can learn something about the absorption spectrum (and hence the composition) of the planet’s atmosphere. If we wait approximately half an orbit we can also search for a second, smaller decrease in light when the planet passes behind the star. If we measure the depth of this eclipse at infrared wavelengths, we can determine the temperature of the planet. Of course, this is only a sampling of the kinds of things that we learn from these systems! They really are an incredibly rich source of information about the worlds outside of our own solar system.

(A observação dos trânsitos, dos eclipses secundários e das fases dos exoplanetas permite extrair informação importante sobre as suas características físicas. Crédito: Sara Seager.)

[AstroPT] – Take planet Jupiter and move it to a tight orbit around the Sun, with a period of, say, 4 days. Assuming our current knowledge of Hot Jupiters, what would Jupiter look like ? What kind of clouds and atmospheric circulation would we see ?

[H. Knutson] – First of all, Jupiter’s clouds would evaporate– instead of water and methane ices, we would see a boiling cloud of extremely hot gases. If there are clouds on Hot Jupiters, they’re made from metal or silicate particles; we don’t normally see these kinds of clouds in planetary atmospheres, but we do see them in cool stars with the same temperatures. The next thing that would happen is that tidal forces would slow the rotation period of the planet down to match its orbital period. Ultimately the planet would become tidally locked, meaning that it always shows the same face to its host star. As the planet’s rotation slows down, the familiar alternating bands of winds we see on Jupiter would merge and grow wider, until only a few were left. One of the very first questions we asked about these tidally locked planets is what happens on the permanently-dark night side– are there winds that carry heat around from the day side and equalize the temperatures, or do you have a boiling hot day side and a freezing cold night side?

[AstroPT] – One interesting development in the study of Hot Jupiters is the finding that some have stratospheres whereas others do not. What causes the stratospheres to form ? How do you use transit observations to determine whether the planet has one or not ? How does having a stratosphere affect the structure and circulation of the atmosphere in a Hot Jupiter?

[H. Knutson] – When we look at the emission spectra of Hot Jupiters (which we can measure using secondary eclipse observations, when the planet passes behind the star), we see some planets have water and other molecular bands in absorption, while in other planets they appear in emission. By default, we expect that temperature decreases with height in planetary atmospheres, so we would normally expect to see absorption features, which are created by a cool, optically thin layer of gas overlying a hotter, optically thick layer deeper down. To get emission features, you have to reverse that pattern, and create a hot, optically thin layer of gas high up in the atmosphere, with a cooler, optically thick layer farther down. The easiest way to do that is to add an extra absorber high up in the planet’s atmosphere, something that’s very good at capturing the energy from the incident starlight, producing extra heating in this layer. In the case of Hot Jupiters, we still don’t know what the extra absorber is– although there are several theories, none of them seems to work very well. There are some hints, though, that the extra absorber is most commonly found in planets that receive relatively little UV flux from their host stars (which destroys many molecules), and that having a carbon-to-oxygen ratio that’s too high (i.e., too much carbon or too little oxygen) can also inhibit the formation of these layers. I’d say the jury is still out on this one, though.

(Um corte vertical da atmosfera de Júpiter. O zero na escala da altitude é, por convenção, medido a partir da camada com pressão atmosférica de 1 bar. As nuvens ocorrem na troposfera, até cerca de 21 quilómetros de altitude. Há várias camadas de nuvens, a altitudes diferentes, devido aos pontos de condensação característicos das diferentes moléculas que as compõem. Note-se como na troposfera a temperatura desce à medida que subimos em altitude. Acima dos 21 quilómetros existem apenas espécies químicas que não foram condensadas nas nuvens subjacentes. No caso de Júpiter, existe metano que (entre outros) absorve fortemente a radiação solar fazendo com que a temperatura aumente com a altitude formando uma estratosfera. A Terra também tem uma estratosfera a partir de uma altitude de aproximadamente 10 quilómetros. A espécie química responsável pela absorção da radiação solar nesta camada é, neste caso, o ozono. Crédito: Encyclopaedia Britannica Inc.)

[AstroPT] – By observing a system during the transit, you can use the fraction of the light of the star that passes through the planetary atmosphere to gain insight into its composition and temperature as a function of height. How do you do this and how difficult are these observations ? When you observe a spectral-line from a given molecule, how do you know what level of the atmosphere it corresponds to ?

[H. Knutson] – These are some of the most challenging observations we make for extrasolar planets, because the signal is so small that we must typically achieve a signal-to-noise of a part in 10,000 or better in order to detect it. In general, we see absorption from the very highest altitudes using this technique, but it can be challenging to determine precisely where in the atmosphere the absorption originates, or what the local conditions are in that region.

(Durante um trânsito, parte da luz da estrela atravessa a atmosfera do exoplaneta. Os átomos ou moléculas na atmosfera absorvem a luz em comprimentos de onda específicos dando origem a linhas espectrais subtis que podem ser detectadas, com alguma dificuldade, em observatórios na Terra ou no espaço. Crédito: D. Sing.)

[AstroPT] – Several transiting Hot Neptunes have also been found. How do these planets compare, in terms of atmospheric composition, type of clouds and circulation, to Hot Jupiters ?

[H. Knutson] – Many of these planets are not only smaller, but also cooler than the typical Hot Jupiters we’ve studied to date. Some also have eccentric (i.e., ellipsoidal) orbits, which means that their atmospheres are not maintained at a constant temperature, but are alternately heated and then cooled as the planet moves through its orbit. They also have higher densities, which implies that unlike Hot Jupiters, they probably have a large rocky or icy core, with a hydrogen/helium envelope on top. These planets likely have clouds and more methane in their atmospheres than Hot Jupiters, but past history has taught us that our predictions for extrasolar planets are often incorrect, so I don’t want to speculate too much!

[AstroPT] – What is the importance of the recent discovery of the transits of 55 Cancri-e ? The available observations give two inconsistent views of the planet, the MOST data pointing to a very dense Super-Earth and the Spitzer data pointing to a border-line Hot Neptune. Given your experience with Warm Spitzer, do you think there is a problem with the MOST data or is there a scenario that subsumes both views ?

[H. Knutson] – This is definitely a topic at the cutting edge of exoplanet science! The MOST team recently put out a revised paper with a new radius estimate more in line with the large Spitzer value, so I believe the consensus has settled on a lower density for this planet. That’s actually quite surprising, because this planet is so hot that it would presumably be difficult for it to hold on to an extended hydrogen/helium atmosphere. In the coming year the Hubble Space Telescope will observe several transits of this planet to search for the tell-tale absorption signature of an extended hydrogen atmosphere in the UV, which should give us a better idea as to whether or not this planet’s atmosphere is being stripped off by its close proximity to its host star.

(Uma fotografia do trânsito de Vénus de 2004, obtida no Dutch Open Telescope (DOT), em La Palma, Canárias. Esta poderia ser também uma fotografia do 55 Cancri-e obtida a partir de uma sonda inter-estelar imaginária. Crédito: DOT Group.)

[AstroPT] – The Spitzer Infrared Space Telescope has been the workhorse for most of the work on exoplanetary atmospheres. What other facilities have the capability to perform the delicate observations required by your work ? If you could ask for a new instrument optimized for atmospheric studies of exoplanets, what would it be ? Do you foresee anything similar being built in the near future?

[H. Knutson] – The Hubble Telescope has also been at the forefront of these observations, and more recently several medium and large ground-based telescopes, including the Canada-France-Hawaii Telescope, the Gran Telescopio Canarias, and the Very Large Telescope in Chile have produced some exciting results. If/when it is built, the James Webb Space Telescope will be a wonderfully sensitive platform for making these kinds of observations in the infrared. It is really hard to beat a space-based telescope, both for sensitivity and stability.

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