Clever Trick Allows Precision Measurement of Exoplanet – Surprisingly High Density Challenges Planet Formation Theories

K2-25b

New detailed observations with NSF’s NOIRLab facilities reveal a young exoplanet, orbiting a young star in the Hyades cluster, that is unusually dense for its size and age. Slightly smaller than Neptune, K2-25b orbits an M-dwarf star — the most common type of star in the galaxy — in 3.5 days. Credit: NOIRLab/NSF/AURA/J. Pollard

Small telescope and inexpensive diffuser key to results.

New detailed observations with NSF’s NOIRLab facilities reveal a young exoplanet, orbiting a young star in the Hyades cluster, that is unusually dense for its size and age. Weighing in at 25 Earth-masses, and slightly smaller than Neptune, this exoplanet’s existence is at odds with the predictions of leading planet formation theories.

New observations of the exoplanet, known as K2-25b, made with the WIYN 0.9-meter Telescope at Kitt Peak National Observatory (KPNO), a Program of NSF’s NOIRLab, the Hobby-Eberly Telescope at McDonald Observatory and other facilities, raise new questions about current theories of planet formation[1]. The exoplanet has been found to be unusually dense for its size and age — raising the question of how it came to exist. Details of the findings appear in The Astronomical Journal.

Slightly smaller than Neptune, K2-25b orbits an M-dwarf star — the most common type of star in the galaxy — in 3.5 days. The planetary system is a member of the Hyades star cluster, a nearby cluster of young stars in the direction of the constellation Taurus. The system is approximately 600 million years old, and is located about 150 light-years from Earth.

Planets with sizes between those of Earth and Neptune are common companions to stars in the Milky Way, despite the fact that no such planets are found in our Solar System. Understanding how these “sub-Neptune” planets form and evolve is a frontier question in studies of exoplanets.

Hyades Star Cluster K2-25

A diagram showing the location of exoplanet K2-25b’s host star in the Hyades star cluster. Credit: NOIRLab/NSF/AURA/Digitized Sky Survey 2

Astronomers predict that giant planets form by first assembling a modest rock-ice core of 5–10 times the mass of Earth and then enrobing themselves in a massive gaseous envelope hundreds of times the mass of Earth. The result is a gas giant like Jupiter. K2-25b breaks all the rules of this conventional picture: with a mass 25 times that of Earth and modest in size, K2-25b is nearly all core and very little gaseous envelope. These strange properties pose two puzzles for astronomers. First, how did K2-25b assemble such a large core, many times the 5–10 Earth-mass limit predicted by theory?[2] And second, with its high core mass — and consequent strong gravitational pull — how did it avoid accumulating a significant gaseous envelope?

The team studying K2-25b found the result surprising. “K2-25b is unusual,” said Gudmundur Stefansson, a postdoctoral fellow at Princeton University, who led the research team. According to Stefansson, the exoplanet is smaller in size than Neptune but about 1.5 times more massive. “The planet is dense for its size and age, in contrast to other young, sub-Neptune-sized planets that orbit close to their host star,” said Stefansson. “Usually these worlds are observed to have low densities — and some even have extended evaporating atmospheres. K2-25b, with the measurements in hand, seems to have a dense core, either rocky or water-rich, with a thin envelope.”

Engineered Diffuser

An example of a 5 cm by 5 cm (2 inch by 2 inch) Engineered Diffuser. Credit: Gudmundur Stefansson/RPC Photonics

To explore the nature and origin of K2-25b, astronomers determined its mass and density. Although the exoplanet’s size was initially measured with NASA’s Kepler satellite, the size measurement was refined using high-precision measurements from the WIYN 0.9-meter Telescope at KPNO and the 3.5-meter telescope at Apache Point Observatory (APO) in New Mexico. The observations made with these two telescopes took advantage of a simple but effective technique that was developed as part of Stefansson’s doctoral thesis. The technique uses a clever optical component called an Engineered Diffuser, which can be obtained off the shelf for around $500. It spreads out the light from the star to cover more pixels on the camera, allowing the brightness of the star during the planet’s transit to be more accurately measured, and resulting in a higher-precision measurement of the size of the orbiting planet, among other parameters[3].

“The innovative diffuser allowed us to better define the shape of the transit and thereby further constrain the size, density, and composition of the planet,” said Jayadev Rajagopal, an astronomer at NOIRLab who was also involved in the study.

WIYN 0.9 Meter Telescope

Sunset at the WIYN 0.9 Meter Telescope at Kitt Peak National Observatory. Credit: KPNO/NOIRLab/NSF/AURA/P. Marenfeld

For its low cost, the diffuser delivers an outsized scientific return. “Smaller aperture telescopes, when equipped with state-of-the-art, but inexpensive, equipment can be platforms for high impact science programs,” explains Rajagopal. “Very accurate photometry will be in demand for exploring host stars and planets in tandem with space missions and larger apertures from the ground, and this is an illustration of the role that a modest-sized 0.9-meter telescope can play in that effort.”

Thanks to the observations with the diffusers available on the WIYN 0.9-meter and APO 3.5-meter telescopes, astronomers are now able to predict with greater precision when K2-25b will transit its host star. Whereas before transits could only be predicted with a timing precision of 30–40 minutes, they are now known with a precision of 20 seconds. The improvement is critical to planning follow-up observations with facilities such as the international Gemini Observatory and the James Webb Space Telescope[4].

Many of the authors of this study are also involved in another exoplanet-hunting project at KPNO: the NEID spectrometer on the WIYN 3.5-meter Telescope. NEID enables astronomers to measure the motion of nearby stars with extreme precision — roughly three times better than the previous generation of state-of-the-art instruments — allowing them to detect, determine the mass of, and characterize exoplanets as small as Earth.

Notes

[1] The planet was originally detected by Kepler in 2016. Detailed observations for this study were made using the Habitable-zone Planet Finder on the 11-meter Hobby-Eberly Telescope at McDonald Observatory.

[2] The prediction from theory is that once planets have formed a core of 5–10 Earth-masses they begin to accrete gas instead: very little rocky material is added after that.

[3] Diffusers were first used for exoplanet observations in 2017.

[4] GHOST, on Gemini South, will be used to carry out transit spectroscopy of exoplanets found by Kepler and TESS. Their target list includes the star K2-25.

More information

This research was presented in a paper to appear in The Astronomical Journal.

The team is composed of Gudmundur Stefansson (The Pennsylvania State University and Princeton University), Suvrath Mahadevan (The Pennsylvania State University), Marissa Maney (The Pennsylvania State University), Joe P. Ninan (The Pennsylvania State University), Paul Robertson (University of California, Irvine), Jayadev Rajagopal (NSF’s NOIRLab), Flynn Haase (NSF’s NOIRLab), Lori Allen (NSF’s NOIRLab), Eric B. Ford (The Pennsylvania State University), Joshua Winn (Princeton), Angie Wolfgang (The Pennsylvania State University), Rebekah I. Dawson (The Pennsylvania State University), John Wisniewski (University of Oklahoma), Chad F. Bender (University of Arizona), Caleb Cañas (The Pennsylvania State University), William Cochran (The University of Texas at Austin), Scott A. Diddams (National Institute of Standards and Technology, and University of Colorado), Connor Fredrick (National Institute of Standards and Technology, and University of Colorado), Samuel Halverson (Jet Propulsion Laboratory), Fred Hearty (The Pennsylvania State University), Leslie Hebb (Hobart and William Smith Colleges), Shubham Kanodia (The Pennsylvania State University), Eric Levi (The Pennsylvania State University), Andrew J. Metcalf (Air Force Research Laboratory, National Institute of Standards and Technology, and University of Colorado), Andrew Monson (The Pennsylvania State University), Lawrence Ramsey (The Pennsylvania State University), Arpita Roy (California Institute of Technology), Christian Schwab (Macquarie University), Ryan Terrien (Carleton College), and Jason T. Wright (The Pennsylvania State University).

7 Comments on "Clever Trick Allows Precision Measurement of Exoplanet – Surprisingly High Density Challenges Planet Formation Theories"

  1. “The prediction from theory is that once planets have formed a core of 5–10 Earth-masses they begin to accrete gas instead: very little rocky material is added after that.”

    I don’t understand why it wouldn’t collect much more rocky material; is it just because it’s already collected all of it in the area of influence?

    Is there some reason that this can’t possibly be the winner of planetary billiards, where perturbations have knocked a more-than-usual number of planetoids and planets out of orbit to be swallowed by K2-25b? It would be unusual to NOT see at least one like that, sooner or later.

    • Torbjörn Larsson | August 9, 2020 at 5:11 am | Reply

      Yes, the main so called core accretion model (not the only formation model) implies just that.

      Yes, something like “planet billiards” can be involved:
      “Given its large core, it should’ve acquired an enormous gaseous envelope. That fact that it didn’t gives rise to several possibilities. One of those possibilities, the authors suggest, is that the rocky core formed through mergers. “To explain its currently observed mass, we surmise that K2-25b could be the product of planet merging events of smaller planetary cores to produce a more massive planet.” That explanation also has the added benefit of accounting for the planet’s orbital eccentricity. They write: “Such a dynamical environment could have excited K2-25b into an eccentric orbit, and K2-25b could be in the process of migrating to a shorter period orbit through tidal interactions with the host star.”” [ https://www.universetoday.com/147318/a-strange-planet-has-been-found-thats-smaller-than-neptune-but-50-more-massive/ ]

  2. Seems at least half the cosmology articles I read “challenge” (or if it conducted itself as a true science, disprove) mainstream theories. The whole state of affairs is ridiculous because literally none of the foundational beliefs of mainstream cosmology seem to ever hold up to new observations, so more and more ad hoc theories have to be improvised to save the beliefs. Just pay attention to how many article titles include “surprising,” “calls into question,” “shocking,” etc etc. Well, if you assume the earth is flat, sure, you’d be saying the same kinds of things about each new discovery about it. So, time to check your assumptions!

    • Torbjörn Larsson | August 9, 2020 at 5:20 am | Reply

      It is of course interesting in a science meritocracy – or even for passive bystanders – to find new science. (And I think you man “reject by testing” since “prove (proof)” is math.) And to check constraints (not ‘assumptions’) as well as other earlier observations continually.

      All the last 2 decades of mainstream cosmology has hold up in ever improving observation in the self consistent cosmology that resulted after the discovery of the cosmic background fluctuation spectra and its characterization. If you read the nearest good encyclopedia on the subjects you would know that instead of effectively trolling the subject [ https://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiation , https://en.wikipedia.org/wiki/Science ].

    • Torbjörn Larsson | August 9, 2020 at 5:21 am | Reply

      It is of course interesting in a science meritocracy – or even for passive bystanders – to find new science. (And I think you man “reject by testing” since “prove (proof)” is math.) And to check constraints (not ‘assumptions’) as well as other earlier observations continually.

      All the last 2 decades of mainstream cosmology has hold up in ever improving observation in the self consistent cosmology that resulted after the discovery of the cosmic background fluctuation spectra and its characterization. If you read the nearest good encyclopedia on the subjects you would know that instead of effectively trolling the subject [ https://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiation ].

    • Torbjörn Larsson | August 9, 2020 at 5:21 am | Reply

      [ That link and this: https://en.wikipedia.org/wiki/Science ].

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