Sunday, September 18, 2022

James Webb's First Exoplanet Photo Release

A bit over two weeks ago, the James Webb Space Telescope team released their first image of an exoplanet; a planet orbiting another star other than our sun. published an update on things today.  The star is known by catalog number HIP 65426, and the planet's designation adds a (lowercase) b: HIP 65426b, resulting in a rather inglorious name.  If you grew up with Star Trek, Star Wars and the many space operas of the last hundred years, you might be disappointed when you find that instead of glorious panoramas of clouds, oceans or vast deserts, what we get is a point source.  Practically one pixel.  Much like the way our eyes see stars, the planet is too small to see but too bright to ignore.  

JWST’s first images of an alien world, HIP 65426b, are shown at the bottom of a wider image showing the planet’s host star. The images were taken at different wavelengths of infrared light. (Image credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI). [NIRCam = Near InfraRed Camera, MIRI = Mid InfraRed Instrument; the numbers presumably are the wavelengths in Angstroms - SiG]

If you're under about 40 years old, you may not have noticed it, but during your lifetime we've entered a new age in astronomy.  As puts it:

Over the past three decades, we have lived through a great revolution — the dawn of the Exoplanet Era. Where we once knew of no planets orbiting distant stars, and wondered whether the Solar System was unique, we now know planets are everywhere.

The official count of planets outside our solar system stands at 5,090 (as of Sept. 15) and the count grows larger every week.  While other extrasolar planets have been photographed, the vast majority have been detected by observing the light from other stars and plotting their brightness versus time; planets are detected by slightly dimming the star when they pass between us and the star, or other perturbations of the star's light, like causing the star's position to wobble.

HIP 65426b is remarkable because the planet is a monster. 

To gather JWST's first direct images of an exoplanet, astronomers turned the telescope towards the star HIP 65426, whose massive planetary companion HIP 65426b was discovered using direct imaging back in 2017.

HIP 65426b is unusual in several ways — all of which act to make it a particularly "easy" target for direct imaging. First, it is a long way from its host star, orbiting roughly 92 times farther from HIP 65426 than the distance between Earth and the Sun. That puts it around 14 billion kilometres from its star. From our point of view, this makes for a “reasonable” distance from the star in the sky, making it easier to observe.

Next, HIP 65426b is a behemoth of a world — thought to be several times the mass of the solar system's biggest planet, Jupiter. On top of that, it was also previously found to be remarkably hot, with temperature at its cloud tops measuring at least 1,200 degrees Celsius.

Now when I see two paragraphs like that, comparing the new planet to two different planets we're familiar with, I see that as a mixed metaphor.  Since I like my mixed metaphors shaken and not stirred, let me point out that since we're comparing the mass of HIP 65426b to that of Jupiter, let's compare distances, too. Instead of saying the planet is 92 times farther away from its star than Earth is from our sun, let's say HIP 65426b is 18 times farther away from its star than Jupiter is from the sun.

While the distance is far in terms of absolute numbers, it's still a tiny separation in angular terms as seen from the Earth/Moon system.  To photograph a much dimmer planet so close to its star, JWST's instruments are equipped with devices like coronagraphs used here on Earth to photograph the Sun's corona by blocking the sun's light and allowing the dimmer light from the corona to build up on the sensor, much as you might put your hand over your eyes to block the sun while looking at something close to it.

The planet surprised the teams analyzing the data.  They determined the mass of HIP 65426b to be roughly seven times that of Jupiter and that the planet is hotter than previously thought, with cloud tops close to 1,400 degrees C.  They also determined that the planet is somewhat smaller than expected with a diameter about 92% that of Jupiter.

To me, one of the highlights of the piece was this little sentence. 

The researchers who led the observations (detailed on the preprint server arXiv - massive pdf warning) found that JWST is performing around ten times better than expected – a result that has astronomers around the globe excited to see what comes next.

This points to the conclusion that the lower limit of what Webb can see and image is very likely smaller than previously expected.  It doesn't have to look for "Super Jupiters" that are larger than our solar system's largest planet.  After all, this photo proves that Webb has already imaged a planet smaller than Jupiter.  Could it image planets the size of Saturn?  The size of the other gas giants in our solar system?  Could it "see" a rocky world the size of Earth?  

My stock answer to the line, "you learn something new every day" was always, "if you're lucky."  I hope we're lucky with Webb.




  1. Excellent news! I'm expecting some very big surprises from this telescope.

  2. Given its mass and temperature, I'd bet heavily that there are continuous nuclear reactions taking place in the core of HIP 65426b. It's often been said of Jupiter that if it were just a little bigger, we'd be seeing the inception of stellar processes there!

    1. An infrared telescope seems like the ideal instrument to see that. If it was continuously fusing hydrogen, I'd expect it to be bright in the IR.

      A brown dwarf? Different sites have different criteria for what a brown dwarf is. One site associated with NASA Goddard doesn't consider candidates for brown dwarf star until they reach 15x the mass of Jupiter, Wikipedia says 13x Jupiter. Either way, "HIP" is still too small.

      There must be some sort of edge condition that could happen when the mass is just right (just wrong?) when the gravity pulls down, fusion starts, but the outward pressures from that reduces the pressure to the point where fusion stops. Then as gravity exerts the pull again, fusion starts again, causing it alternately puff out, then shrink, puff out, then shrink. That would interesting to see.