HD 63935: Two Sub-Neptunes and Maybe More

Designations like HD 63935 b and c don’t exactly roll of the tongue.

Although with a little work I might pronounce them “trippingly on the tongue,” as Hamlet put it.

Maybe saying “sixty five ninety three five bee and cee” would do the trick. Then again, maybe not. I thought, briefly, of calling HD 63935, HD 63935 b and HD 63935 c “Sam, Fred and Chuck;” but thought better of it.

At any rate, I’d been catching up on ‘exoplanet’ notes from the last year or so when I read about the HD 63935 planetary system.

HD 63935’s known planets there, sub-Neptunes, should help scientists learn more about how planets form. Or, rather, observing them and analyzing those observations should.


Defining and Detecting Planets

We’ve known about five planets since long before we started keeping written records.

Six, maybe, if you count Uranus.

Seven, including Earth, but we didn’t start thinking of our world as a “planet” until recently. Then there’s Earth’s moon and the sun, and that’s another topic. Topics.

How we define “planet” has shifted, most recently in 2006, when the International Astronomical Union issued a definition that’s still debatable. And debated.

Today’s telescopes let us see the Solar System’s planets, and about two dozen exoplanets: planets orbiting other stars.

The vast majority of exoplanets, however, aren’t directly visible. Not yet.

We know they’re there because they affect the stars they orbit.

Some are massive enough to move their stars toward and away from us as they orbit, or move the star back and forth across the sky.

Others pass between us and their star during each orbit. I’ll be talking about two of these transiting exoplanets.1


HD 63935’s Twin Sub-Neptunes

Scarsdale et al's Figure 4: transits of HD 63935 b and c. (2021)
(From Scarsdale et al., via Phys.org, used w/o permission,)
(“Phase-folded transits of HD 63935 b and c.”
(Phys.org.))

Astronomers discover twin sub-Neptune exoplanets orbiting nearby star
Tomasz Nowakowski , Phys.org (October 20, 2021)

“By analyzing the data from the TESS-Keck Survey (TKS), an international team of astronomers reports the detection of two almost identically sized sub-Neptune exoplanets orbiting a nearby star. The newly found alien worlds, designated HD 63935 b and HD 63935 c, are about three times larger than the Earth. The finding is detailed in a paper published October 13 on the arXiv pre-print server.

“NASA’s Transiting Exoplanet Survey Satellite (TESS) is conducting a survey of about 200,000 of the brightest stars near the sun with the aim of searching for transiting exoplanets. So far, it has identified over 4,500 candidate exoplanets (TESS Objects of Interest, or TOI), of which 161 have been confirmed so far….”

HD 63935 is about 159 light-years out, in the general direction of Procyon. It’s a tad cooler than our star: 5,534 °K or maybe 5,560 °K, compared to Sol’s 5,772 °K.

The lower number for HD 63935 is from The Extrasolar Planet Encyclopaedia’s page, the arXiv paper’s number is 5,560 °K.

“K” temperatures are degrees Kelvin. The Kelvin temperature scale starts at absolute zero. Comfortable room temperature is 294 °K, eggs fry at 343 °K or thereabouts, and a nicely-burning hearth fire may be around 534 °K.

Getting back to HD 63935, that star is slightly cooler than ours, not quite a tenth as massive and chemically similar. It’s not a ‘Solar twin,’ but it’s close.2

Designations: A Digression

The Winter Triangle: Procyon, Betelgeuse and Sirius. From Akira Fujii; via Hubble Space Telescope, ESA, NASA; used w/o permission.As far as I know, neither HD 63935, HD 63935 b nor HD 63935 c have names yet.

But they’ve got other designations.

I talked about star names and designations last month.3

Basically, now that astronomers are studying far more than the 10,000 or so stars we can see without telescopes, catalog numbers and other designations are more convenient than names.

The planet HD 63935 b, for example, is also TOI 509.01; and the star is TIC 453211454.

The “HD” in their designations stands for Henry Draper Catalogue, which lists spectroscopic classifications for about a quarter-million stars.

HD 63935 is also known as HIP 38374 and TIC 453211454. “HIP” stands for the Hipparcos catalog: data from ESA’s Hipparcos observatory, first published in 1997.

The ESA project’s named after Hipparcus. He was an astronomer, geographer, and mathematician who lived when Mithridates I was putting Parthia on the map. We lost his star catalog sometime during the two millennia that’s elapsed since then.

Finally, TIC stands for Third International Conference, Titanium Carbide, Thermal Imaging Camera and Thames Innovation Centre.

But in this context TIC means TESS Input Catalog.4

Twin Sub-Neptunes: Unlike Anything in the Solar System

Goddard Space Flight Center's illustration of transmission/absorption spectroscopy.
(From NASA’s Goddard Space Flight Center, used w/o permission.)

The diameters of HD 63935 b and c are about 2.99 and 2.9 times Earth’s. That’s big, but not as big as Neptune’s 3.883 Earth-diameters.

On the other hand, HD 63935 b has only around 10.8 times Earth’s mass, which makes its density 2.2 g/cm3. Give or take.

HD 63935 c is smaller but heavier, with around 11.1 Earth masses. Its density is about 2.5 g/cm3.

Neither is nearly as dense as Earth — 5.514 g/cm3 — but they’re more tightly packed than Neptune’s 1.64 g/cm3.

And they’re much warmer than either Earth (287 °K) or Neptune (72 °K where air pressure is like Earth at sea level).

HD 63935 b’s calculated ‘surface’ temperature is between 884 and 938 °K. HD 63935 b’s is a bit cooler, 663 to 705 °K. By comparison, the Solar System’s Mercury surface temperatures max out at around 700 °K at the equator.

That’s because their sun is nearly as bright as ours, and both their orbits are smaller than Mercury’s. HD 63935 b orbits the star every 9.06 days, while a year for HD 63935 c 21.4 days long. Earth’s 24-hour days, that is.5

Exploring the “Radius Cliff”

Fulton and Petigura's exoplanet radius distribution (2018), illustrated by Edwin S. Kite et al. to show 'radius cliff' at ca. 3 Earth-radii. (2019)Since they’re smaller than the Solar System’s Neptune, the recent paper calls HD 63935 b and c “sub-Neptunes.”

Although the label’s accurate by current standards, neither is particularly like Neptune. Or Earth.

The Solar System’s Uranus and Neptune are almost, but not quite, twin planets: almost exactly four times Earth’s diameter and 3.883 Earth-diameters, respectively.

Oddly enough, we’ve been finding a whole lot of planets less than three Earth-diameters, but not nearly as many in the Neptune-Jupiter range. Not with short-period orbits, at any rate.

Scientists have been calling this discontinuity the “radius cliff.”

One of the best, or least-improbable, explanations is that larger planets have a sort of plateau in their growth curves. If this is the case, then when a planet’s core and atmosphere reaches about three Earth-diameters, pressure at the surface of its magma ocean lets the magma start absorbing hydrogen.

The planet keeps on gathering gas, but won’t get bigger until the magma’s saturated.6

Or maybe there’s another explanation. If so, I figure we’ll discover it as we collect more data: and review what we’ve already learned.

“Space Oddities…” — or — Studying Starlight

NASA/STScI's illustration, showing how absorption spectroscopy works. (1998)
(From NASA/STScI, via Wikimedia Commons, used w/o permission,)

Light from HD 63935 travels some 159 light-years before reaching us, which is almost next door or unimaginably distant, depending on what distance scale is in play.

And that’s yet another topic.

The point is that HD 63935 isn’t a particularly dim star and it’s close enough to for spectroscopic analysis. “Spectroscopic analysis” is a five-dollar phrase that means taking a look at the colors in starlight.

And, since the two sub-Neptunes orbiting HD 63935 pass between their sun and our planetary system, scientists can measure what happens to light passing through their atmospheres. Assuming that they have atmospheres, which seems like a safe bet.

Three of the scientists who published the paper discussed in that Phys.org article made a case for follow-up observations back in January.7

  • “HD 63935’s Space Oddities: Two Atmospheric Targets in Sparse Regions of Mass-Radius Space”
    Nicholas Scarsdale, Joseph M. Akana Murphy, Natalie M. Batalha; American Astronomical Society meeting #237 (January 2021)

“Space oddities” is hardly an original phrase, and far from the erudite appellations bestowed upon many of yesteryear’s documents of natural philosophy.

On the whole, I’ve enjoyed watching scientists unstarching their collars in recent decades, and that’s yet again another topic.

Worth a Closer Look

Nicholas Scarsdale et al.'s chart of radial velocity points for HD 63935. (2021)With two sub-Neptunes, HD 63935 is already a two-for-one research opportunity.

But there’s a good chance there’s at least one more planet in that system.

After Scarsdale et al. sorted out radial velocity measurements attributable to HD 63935 b and c, the remainder looked a lot like a wobble made by a third planet.

Or maybe it’s the star’s surface bouncing, but these scientists figure that’s not likely.

The radial velocity remainders probably aren’t linked to the star’s rotation period, 30 to 35 days, either.

On the other hand, Scarsdale et al. didn’t find reasonably certain evidence of a third planet. Which, if it’s there, doesn’t produce effects that would affect the data they did analyze.8

Bottom line? HD 63935 b and c are high-value targets for scientists, and their planetary system may include more worlds for us to study.


Perspectives and Paying Attention

NGC 4848 and other galaxies, image by Hubble/ESA.

“Indeed, before you the whole universe is as a grain from a balance, or a drop of morning dew come down upon the earth.”
(Wisdom 11:22)

We’ve learned a bit about this universe during the two millennia that have passed since those thoughts were recorded.

But as I see it, they’re still true. From God’s viewpoint, a drop of morning dew and our world of planets, stars and galaxies are — all pretty much the same size.

Which, I suppose, could be a scary thought.

Or a reassuring one, since:

“Our God is in heaven
and does whatever he wills.”
(Psalms 115:3)

So, again as I see it, God is large and in charge. And if we pay attention to this wonder-filled creation, we can learn a little more about the Almighty.

“For from the greatness and the beauty of created things their original author, by analogy, is seen.”
(Wisdom 13:5)

And that, I think, is a good idea:


1 Planets past and present:

2 Units of measure and stars:

  • Wikipedia
  • TKS V. Twin sub-Neptunes Transiting the Nearby G Star HD 63935
    Nicholas Scarsdale, Joseph M. Akana Murphy, Natalie M. Batalha, Ian J. M. Crossfield, Courtney D. Dressing, Benjamin Fulton, Andrew W. Howard, Dnaiel Huber, Howard Isaacson, Stephen R. Kane, Erik A. Petigura, Paul Robertson, Arpita Roy, Lauren M. Weiss, Corey Beard, Aida Behmard, Ashley Chontos, Jessie L. Christiansen, David R. Ciardi, Zachary R. Claytor, Karen A. Collins, Kevin I. Collins, Fei Dai, Paul A. Dalba, Diana Dragomir, Tara Fetherolf, Akihiko Fukui, Steven Giacalone, Erica J. Gonzales, Michelle L. Hill, Lea A. Hirsch, Eric L. N. Jensen, Molly R. Kosiarek, Jerome P. de Leon, Jack Lubin, Michael B. Lund, Rafael Luque, Andrew W. Mayo, Teo Močnik, Mayuko Mori, Norio Narita, Grzegorz Nowak, Enric Pallé, Markus Rabus, Lee J. Rosenthal, Ryan A. Rubenzahl, Joshua E. Schlieder, Avi Shporer, Keivan G. Stassun, Joe Twicken, Gavin Wang, Daniel A. Yahalomi, Jon Jenkins, David W. Latham, George R. Ricker, S. Seager, Roland Vanderspek, Joshua N. Winn; The Astronomical Journal (Submitted October 13, 2021) via arXiv
  • The Extrasolar Planet Encyclopedia
  • Fireplace Safety
    Adapted from Shawn Shouse, Iowa State University Extension Field Specialist/AG Engineering; Backyards & Beyond; The University of Arizona

3 Some stars have names, many don’t:

4 Names, catalogs and descriptions:

5 Introducing the HD 63935 planetary system; and, for comparison, Mercury:

6 Background: planets and magma oceans:

7 Starlight and science:

8 Looking ahead:

About Brian H. Gill

I was born in 1951. I'm a husband, father and grandfather. One of the kids graduated from college in December, 2008, and is helping her husband run businesses and raise my granddaughter; another is a cartoonist and artist; #3 daughter is a writer; my son is developing a digital game with #3 and #1 daughters. I'm also a writer and artist.
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