With Hubble, astronomers use UV light for first time to measure a still-forming planet’s growth rate

In 2018, the exo-planet PDS 70b was observed using the Very Large Array.  It’s discovery instantly placed it at the top of observation requests and telescope time for one quite profound reason: the exoplanet was still forming.

For the first time, a still-accreting planet had been discovered, providing astrophysicists a unique opportunity to study how planets form with real-time observations.  But one pesky problem existed: PDS 70b was far too close to its parent star for the usual exoplanet observational techniques to allow researchers to measure the planet’s growth rate.

Now, for the first time ever using UV-band observations, a group of astrophysicists working with the Hubble Space Telescope’s Wide Field Camera 3 have produced the first measurement of PDS 70b’s current growth rate.

The exoplanet in question orbits the star PDS 70 — a young, 10 million year old, K5 spectral type, low-mass T Tauri star located approximately 370 light-years from our solar system in the constellation Centaurus.

A 140 AU-wide (1 Astronomical Unit , or AU, is equal to the average distance of Earth from the Sun: approximately 149 million kilometers) accretion disk around the star was confirmed in 2006, with an approximately 65 AU gap in that disk found in 2012.

The gap instantly intensified interest in PDS 70, as large gaps in young star systems’ accretion disks are usually an indication of forming planets according to models of stellar system development.

PDS 70b seen in this image from the Very Large Telescope. The planet stands out as a bright point to the right of the center of the image. (Credit: ESO, VLT, André B. Müller)

In 2018, exoplanet PDS 70b was found in a 119.2 year orbit located approximately 20 AU from its parent star using the Very Large Telescope in Chile. The massive Jupiter-like planet, itself more than five times Jupiter’s mass, was too close to its parent star to be observed in the ways necessary to discern its current accretion (or growth) rate.

“To study this specific planet, it needs UV information,” said Dr. Yifan Zhou, Postdoctoral Fellow, McDonald Observatory, University of Austin and lead author on the new study on PSD 70b’s accretion measurement, in an interview with NASASpaceflight.

“Hubble is basically the only telescope that can do this work” due to the detail and precision of the observations,” noted Dr. Zhou.

Using the orbital Hubble Space Telescope to observe the PDS 70 system gave Zhou et al. their best chance of seeing the exoplanet in the UV wavelength.  However, “the non-ideal part of Hubble compared to ground-based telescopes [is that] Hubble has a very small mirror.  It’s only 2.4 meters.”

The smaller the mirror, the less sharp the image.  To account for this, the team employed multiple techniques — some for the first time with UV observations — to pull the exoplanet from the data while ensuring they were not detecting a false positive.

“One very important concept here, the application we use in this observation is called angular differential imaging.  You take the image with different position angles, and your PSF, the point spread function structure is rotating with the telescope but your planet is staying in the same position,” related Dr. Zhou.

“We can rotate the two images to have their point spread function match with each other, and when we subtract [them] from each other, your astrophysical signal, or planet signal, stays there and you remove all of your contamination from the star.”

“So that’s a very important technique we used here.  It was developed in ground-based observations of exoplanets,” added Dr. Zhou.

Typically, two angular differential imaging positions are used for such observations.  However, for this investigation, 18 different angles were required to gather the needed information. 

To ensure the data didn’t return a false positive, interference showing an exoplanet where there isn’t one, Zhou et al. used artificial signals purposefully added to the images to ensure they were seeing a real exoplanet.   “At the very first stage, we [inserted] an artificial signal that we [knew was] there.  We inserted it into the images to see if after all these types of image processing we [could] recover [it],” said Dr. Zhou.  “And we recovered it, so that gave us additional confidence we were seeing a real signal.”

Observations with Hubble occurred in two main wavelengths used for the final investigation in conjunction with various filters across 18 orbits.  Each orbit included ten, 120 second UV F336W band exposures and nine 20 second F656N (for hydrogen-alpha, or Hα, emission line) exposures using Wide Field Camera 3.

In total, 21,600 seconds of observations in the F336W band and 3,240 seconds of information in the F656N band were collected.  After working with the data, Zhou et al. confirmed the detection of the exoplanet PDS 70b in UV. 

For the first time, the UV information provided a clear look at the current accretion process taking place at PDS 70b.

After its initial discovery, follow-up observations found PDS 70b likely had a circumplanetary disk of material… just as planetary formation models predicted it would.  The new UV investigation shed further light on the exoplanet’s disk, which itself proved useful in determining the processes still governing PDS 70b’s growth.

The Hα emission lines from PDS 70b observed by Zhou et al. clearly showed active accretion as Hα emissions occur as material, following a forming planet’s magnetic field lines, flows into the planet and is heated in the process — creating a hot shock.

Temperatures of hydrogen atoms in the gas and material being pulled into the planet therefore increase to the point where the atoms are excited and their single electron moves from the second to the third energy state.  When the electron falls back to the second energy level, an Hα emission is produced.

However, a puzzling result from the analysis was the final measured accretion rate, which was found to be: M = 1.4 ± 0.2 x 10-8 MJupyr-1Put another way, under its current accretion rate, it would take PDS 70b one million years to accrete 1/100th of Jupiter’s mass.

And that’s lower than super-Jupiter gas giant planet formation models predict.

PDS 70b, seen by Hubble. (Credit: Joseph DePasquale, STScI)

Zhou et al. are quick to caution that their calculations are a snapshot in time.  Additional observation, multi-decade, multi-century observations will reveal if accretion rates fluctuate greatly over time as planets go through growth spurts, so to speak, followed by periods of less active formation or if “Hα production in planetary accretion shocks is more efficient than [previous] models predicted, or [if] we underestimated the accretion luminosity/rate,” noted Zhou et al. in their paper published in April 2021 issue of The Astronomical Journal.

 The team further noted, “By combining our observations with planetary accretion shock models that predict both UV and Hα flux, we can improve the accretion rate measurement and advance our understanding of the accretion mechanisms of gas giant planets.” 

Moreover, as Dr. Zhou related to NASASpaceflight, additional observations will also reveal how much of the circumplanetary disk will accrete to the planet and how much of it will remain to form moons.

“After the planet accretion is finished, there’s leftovers in the circumplanetary disk.  Those materials, they congregate.  Now, in terms of discovering them, my expectation is that it’s very, very challenging.”

Not only is discerning an exomoon around an exoplanet incredibly difficult, alignment is also key.  “We need to have the planet aligned with the star and then the moon aligned with the planet,” related Dr. Zhou.

While there are no confirmed exomoons to date, a few candidates have been proposed, and the potential – as technical advances – that PDS 70b could provide a close-to-home look at exomoon development remains.

To this end, Dr. Zhou looks forward to the pending launch of the James Webb Space Telescope and its 6.5 meter mirror and infrared imaging capability.

“For James Webb, we will have the first opportunity to probe the actual disk that is being accreted onto the planet.  And, actually, PDS 70b is a prime target for multiple James Webb programs that already have the guaranteed time observation.  So [those teams will] observe this planet in multiple wavelengths,” noted Dr. Zhou.

The PDS 70 has two confirmed exoplanets, 70b and 70c, the latter of which was not seen in the data.

(Lead image credit: NASA, ESA, STScI, Joseph Olmsted)

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