James Webb Space Telescope’s Stunning Results for Transiting Exoplanet WASP-96b (2 of 5)

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Light curve of WASP-96 b as produced from JWST data during commissioning operations.

In our presentation of the NASA-hosted event opening the James Webb Space Telescope to the world’s astronomical community, we discussed one of the five commissioning targets presented to the public. That choice to open this 5-article series was SMACS 0723, a distant galaxy cluster receding at 29% the speed of light.

In this article, we’ll present our choice for the 2nd of the 5 targets, WASP-96 b, a transiting exoplanet, what can accurately be referred to as a ‘hot Jupiter‘. Hot because of it’s 0.04 AU (1 Astronomical Unit = the Earth-Sun distance) distance from the host star and Jupiter because its mass and radius are comparable to Jupiter’s.

Before we begin though, it’s necessary to briefly describe the naming convention for this class of astronomical object. The “root” or primary name of the planet is based on the star’s name, in this case WASP-96, named for a particular exoplanet survey of stars. WASP is the acronym for Wide Angle Search for Planets and the host star is the 96th entry in the catalog. The planet found is the first and is designated as ‘b’. If there are additional planets in this system, they will be named ‘c’, ‘d’, ‘e’, etc, accordingly in order of discovery and in distance from the host star.

WASP-96

In a view to the south from the NE US, WASP-96 just skirts the southern horizon. The star will be most favorably observed at locations south of the equator and, at 1152 light years, will require at least a 15 cm (6″) telescope to be seen at all.

WASP-96 is a 12th magnitude star, 1152 light years distant in the southern constellation Phoenix and requires at least a 15 cm diameter telescope to be seen at all.

Before we discuss planet b of the system, the ‘hot Jupiter‘, it is instructive to discuss the host star, WASP-96. This star is classified as a G8 star, the same broad spectral classification as the sun, a ‘G’-class star. The sun is a G2V star, a designation that describes it as slightly hotter and more luminous than WASP-96. Within spectral class, a smaller number indicates a hotter effective temperature and, with a designation of ‘2’ within this class, the sun is slightly hotter at 5780 Kelvin compared to 5500 Kelvin for WASP-96.

The ‘V’ designates the ‘Luminosity’ class, identifying the sun as a main sequence star. This ‘Main Sequence‘ designation describes a star in balance between the inward crush of gravity and the outward pressure of the super-heated plasma, powered by thermonuclear fusion reactions in its core. In the case of the sun, 610 million metric tons of hydrogen are transmuted into helium every second.

Luminosity class of WASP-96

We know its Spectral class is G8, but what is its luminosity class?

The Spectral class describes the effective temperature of the star’s photosphere, the disk we can ‘see’ (it’s really the balance point between gravity and outward gas/radiation pressure). Since the star’s absolute total brightness, its luminosity, depends on a second factor, the star’s physical radius, we need to know that to fully describe the absolute radiant output of the star. The luminosity class is such a designation.

Since both of these components of a star’s radiant output are empirically determined (measured) by ground-based or space-based (orbiting observatories) platforms, some of the data may be inaccurate or uncertain and require confirmation and verification. In the case of WASP-96’s spectral class, 3 separate studies confirm its temperature as 5500 Kelvin and thus, the G8 designation is confirmed. The star’s luminosity class is another matter, however.

One of the few sources that hosted this data lists it as ‘D’, giving the complete classification of the star as ‘G8 D’, a designation that is clearly erroneous. A luminosity class of ‘D’ is reserved for White Dwarfs, something this star most certainly is not!

Based on its confirmed radius of 1.1 solar and age of 8+ billion years, it is in an evolved state and leaving the main sequence, never to return. It’s on its way to become a sub-giant, the next phase in stellar evolution after the main sequence for solar-mass stars. This evolutionary track applies for all such stars as they deplete their compliment of hydrogen fuel. As such, the correct and complete classification for WASP-96 would be G8-IV.

Stellar Evolution

Once a star evolves off the main sequence, it never returns. For stars in the narrow class range between G2 (the sun) and G8 (WASP-96), they cool as they expand, age and evolve, on their way to become sub-giants and giants eventually. As they expand, they cool (energy is distributed over a greater surface area), their luminosity increases as they expand. Luminosity or total radiant energy increases as the second power of the radius and the fourth power of the temperature. Interestingly, the change in luminosity is more sensitive to changes in size than temperature. Why am I pointing this out?

The 5500 Kelvin effective temperature of WASP-96 was not its main sequence effective temperature. It has evolved to become a G8 star now and was hotter and smaller previously when it was on the main sequence. It was probably a G5 or maybe even a G2 star such as the sun. How is this possible? The G8 designation was assigned now, mostly according to the star’s current effective temperature. Don’t forget as a star ages off the main sequence, it expands -and cools! The present radius of WASP-96 has expanded from its main sequence radius to a radius 10% greater than the sun. As it expands it also cools. These combined changes suggesting it was hotter and smaller when it was on the main sequence.

Its advanced age of 8+ billion years is also consistent with this proposition. WASP-96 has left the main sequence and is evolving towards a sub-giant.

I’m spending considerable digital ink here for a specific reason, to give us a glimpse into our own, distant future. We’re looking forward in time, 3.5 billion years, to when the sun evolves off the main sequence. By understanding how this star and its planetary environment evolved, new insights emerge of how our own system will evolve.

WASP-96 b
WASP-96 b was discovered through the Wide Angle Search for Planets program. It was chosen as one of the first objects of study because of its clear skies! Literally! Why so? As explained by the JWST team during the July 12th media briefing:

What’s special about WASP-96b is that it isn’t cloudy – the new spectrum shows some evidence of clouds and haze, but not much. That’s good because it allows the starlight to shine right through the atmosphere and for us to analyze it without being blocked by lots of clouds.

So what did JWST’s instruments (NIRIss) find in the atmosphere? Water and a lot of it! Not as a liquid but as a vapor, a super-heated vapor at 1350 kelvin! It’s instructive to consider the following remarks about the discovery:

The labeled peaks in the spectrum indicate the presence of water vapor. The height of the water peaks, which is less than expected based on previous observations, is evidence for the presence of clouds that suppress the water vapor features. The gradual downward slope of the left side of the spectrum (shorter wavelengths) is indicative of possible haze. The height of the peaks along with other characteristics of the spectrum is used to calculate an atmospheric temperature of about 1350°F (725°C).

This is the most detailed infrared exoplanet transmission spectrum ever collected, the first transmission spectrum that includes wavelengths longer than 1.6 microns with such high resolution and accuracy, and the first to cover the entire wavelength range from 0.6 microns (visible red light) to 2.8 microns (near-infrared) in a single shot.

Stars as Blackbodies

The indicated temperature of the water vapor is consistent with the prevailing temperature at the planet’s distance from WASP-96. Modeling WASP-96 and its environs using Stephan-Boltzmann’s law yields a prevailing temperature of 1800 kelvin at 0.04 AU from WASP-96. This is to say, we’ve modeled the star as a blackbody (stars are modeled as blackbodies – this is a topic for another discussion). As well also, this is not to say that this is the temperature at the planet’s surface or even in the cloud decks. Why? Because the planet -with its atmosphere- is also modeled as a blackbody. This is important! The energy received from the star (the prevailing temperature) is in thermodynamic equilibrium with the planet and its atmosphere. It’s hotter above the atmosphere than at the surface or even in the cloud decks. Let us consider the earth and its near-space environs to help us better understand this.

Modeling the sun using Stephan-Boltzmann’s law, we find a prevailing temperature of 394 Kelvin at the Earth-sun distance (1 AU). This is 19 degrees above the boiling point of water. Clearly, we don’t experience these temperatures at the surface! The atmosphere acts as a shield and a blanket, protecting us from these extremes in temperature. It also protects us from other lethalitys of earth’s near-space environment (a topic for another discussion). The same can be said of WASP-96b’s atmosphere. 1800 Kelvin is approaching plasma temperatures, the point where water would begin to dissociate. Hydrogen bonds, the electrostatic force that holds the water molecule together, won’t hold above 2270 K. The planet and its atmosphere are in thermodynamic equilibrium with the energy received. The indicated temperature of 1350 Kelvin of WASP-96b is consistent with this modeling, much as the earth near-space environment is.

It is interesting to ponder this planet orbiting a smaller, hotter star during a previous evolutionary epoch. As a result of the current late main-sequence to sub-giant evolutionary changes, the star’s radius has increased to 1.1 solar. Given its age and effective temperature, it’s reasonable to extrapolate back in time and assign a main-sequence classification of G5-V. In doing so, we note an effective temperature of 5660 K and a radius of 0.98 solar. It has since increased in size by 12% to 1.1 solar or 1,533,400 km in diameter.

This increase in size has reduced the distance to the planet bringing it to 0.04 AU (6,000,000 km). At this distance, WASP-96 subtends an angle of 14 degrees on the sky!  This is 28x the sun’s apparent size as seen from earth!

As the eons go by, it only gets worse for WASP-96b. The host star will continue its evolutionary track away from the main sequence. Within a few hundred million years, it will completely engulf the planet.

A Transiting Exoplanet

WASP-96b is a transiting exoplanet as seen from earth. The only way JWST can analyze planetary atmospheres is if the target system is favorably aligned. That is, if the planet transits across the the host star’s disk, also known as a ‘transit‘. By measuring the drop in the received light from the star, we can deduce the planet’s size and orbital period. The length of the transit allows us to deduce the planet’s orbital period. As the planet passes in front of the star, we analyze the light as it passes through the planet’s atmosphere. The spectral changes -compared to the star’s base spectrum- will reveal the composition of the planet’s atmosphere. And this is exactly what JWST’s NIRIss instrument did!

Light curve of WASP-96 b as produced from JWST data during commissioning operations.


Attribution

”The Early Release Observations and associated materials were developed, executed, and compiled by the ERO production team: Hannah Braun, Claire Blome, Matthew Brown, Margaret Carruthers, Dan Coe, Joseph DePasquale, Nestor Espinoza, Macarena Garcia Marin, Karl Gordon, Alaina Henry, Leah Hustak, Andi James, Ann Jenkins, Anton Koekemoer, Stephanie LaMassa, David Law, Alexandra Lockwood, Amaya Moro-Martin, Susan Mullally, Alyssa Pagan, Dani Player, Klaus Pontoppidan, Charles Proffitt, Christine Pulliam, Leah Ramsay, Swara Ravindranath, Neill Reid, Massimo Robberto, Elena Sabbi, Leonardo Ubeda. The EROs were also made possible by the foundational efforts and support from the JWST instruments, STScI planning and scheduling, and Data Management teams.”



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