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Spectroscopy
Although right now I'm more excited about ESPRESSO's radial velocity measurements, so I'm listening to This Kiss, her song about measuring "centrifugal motion" on "a rooftop under the sky".
Title text: Although right now I'm more excited about ESPRESSO's radial velocity measurements, so I'm listening to This Kiss, her song about measuring "centrifugal motion" on "a rooftop under the sky".

Explanation

This comic mixes the method of using spectroscopy to detect oxygen on exoplanets (planets outside our Solar system) with the lyrics for the Faith Hill song "Breathe" (listen to "Breathe" on YouTube).

From the lyrics:

I watch the sunlight
dance across your face
I can feel you breathe

In the comic the word feel has been changed to see. The two first panels are one line in the song. The last line is from the chorus and is repeated five times during the song, although not right after the first two lines.

In the first and second panel the singer examines the spectra of a remote planet by watching the sunlight during the transit of the planet as this sunlight dances across the planet's face. Finally we determine that breathable oxygen exists. Since we cannot (as Faith can) feel the planet we have to see it. And by doing this I can see you breathe.

Measuring the light output of stars (spectra) we are able to determine a number of details of the star, including rotation, relative radial velocity, chemical composition, temperature, and to some degree, distance and size. When a planet, as pictured, moves between the star and the observer, then by looking at the spectrum received, the viewer is able to determine the contents of the planet's atmosphere from the specific wavelengths of light that are absorbed in this. If it turns out that the atmosphere absorbs the lines corresponding to molecular oxygen (O2) this is a clear indication that the planet has large quantities of breathable oxygen (but not necessarily life). However, there must be oxygen in large amounts in the atmosphere to sustain most of the life forms that we know of here on Earth (though not all). It is thus clear why Randall would be interested in exoplanets with oxygen.

This comic came out four days after this article about NASA's New NExSS Initiative. NASA will search for signs of life on other planets, for instance by using "the light passing through the atmospheres of these exoplanets". And they "will study chemicals that have been detected on other worlds, such as oxygen and methane, to see if they were produced by biology".

The title text refers to determining radial velocity in the ESPRESSO program (Echelle SPectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations). By noting that the radial velocity of the star changes slightly as the planet that orbits it moves around the star (centrifugal acceleration), the ESPRESSO program should be able to detect the masses of planets as they are moving towards the Earth in their orbit around their distant stars. The ESPRESSO program is so precise that it should be able to detect planets as small as Earth and the other of the Solar system's inner planets.

Randall is now even more excited about ESPRESSO than he is about the oxygen levels, because it is now possible to detect these "very" small planets. So he is no longer listening to "Breathe", but to another Faith Hill song: "This Kiss" (listen to "This Kiss" on YouTube).

From the lyrics:

It's centripetal motion
On the rooftop under the sky

The first line is part of the chorus and it is repeated four times. Randall has changed the main word to centrifugal though. There is, however, disagreement on-line whether it is centripetal or centrifugal, but maybe this is because people generally do not know the word centripetal? (That there may be many hits with one specific version over another may come from the fact that lyrics services copy/paste...) I would follow the first part of the comic, that Randall has changed a main part of the lyrics.

The second line is not sung in connection with the chorus, and it is only changed a bit, so the is changed to a. Also the "on" which is part of his line here, is not part of the quoted line in the title text.

The song is not about measuring but, of course, about "The Kiss". Since the ESPRESSO is part of the Very Large Telescope, it is located on the Cerro Paranal mountain in the Atacama desert in Chile at an elevation of 2,635 meters (8,645 ft.) above sea level. So it could be said that it is measuring on a rooftop under the sky. Although it is radial velocity it measures not centrifugal motion, the object it does measure will all be experiencing this fictitious force (also see 123: Centrifugal Force), as the planets are in orbit around a star.

Randall has previously made several references to exoplanets in his comics, most notable are the two comics with the same name: 786: Exoplanets and 1071: Exoplanets. The latter comic came out when there were exactly 786 exoplanets found. Today more than 1900 have been discovered (1915 as of Wikipedia on the release day of this comic), much more than twice that amount. And now they can find even smaller planets, and detect the atmosphere. Much have happened since the first exoplanet comic came out in 2010.

Transcript

[A dark panel with a bright star in the center. To the left a planet (drawn as a new moon) approaches the star. Text is written above in white with two musical notes, one on each side of the text.]
I watch the sunlight
[Same image but now the planet transits the star. Small lines around the planet indicate the atmosphere, as seen from the light from the star passing through it. Text is again written above in white with two different musical notes, one on each side of the text.]
Dance across your face
[A white frame with a black line. It Is the spectrum of the planets atmosphere. Two distinct absorbtion peaks are visible. The first one is labeled with an arrow. Text is again written above, now in black, with two, again, different musical notes, one on each side of the text.]
I can see you breathe
Label: O2
[Below the panels is the following caption:]
Faith Hill on exoplanet spectroscopy


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