Surprisingly dense exoplanet challenges planet formation theories
Small telescope and inexpensive diffuser key to results

Date:
August 4, 2020
Source:
Association of Universities for Research in Astronomy (AURA)
Summary:
New detailed observations 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.



FULL STORY ==========================================================================
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.

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." 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.

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: no more rocky
material is added.

[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.


========================================================================== Story Source: Materials provided by Association_of_Universities_for_Research_in_Astronomy_ (AURA). Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Gudmundur Stefansson, Suvrath Mahadevan, Marissa Maney, Joe
P. Ninan,
Paul Robertson, Jayadev Rajagopal, Flynn Haase, Lori Allen,
Eric B. Ford, Joshua Winn, Angie Wolfgang, Rebekah I. Dawson,
John Wisniewski, Chad F.

Bender, Caleb Can~as, William Cochran, Scott A. Diddams,
Connor Fredrick, Samuel Halverson, Fred Hearty, Leslie Hebb,
Shubham Kanodia, Eric Levi, Andrew J. Metcalf, Andrew Monson,
Lawrence Ramsey, Arpita Roy, Christian Schwab, Ryan Terrien, Jason
T. Wright. The Habitable-zone Planet Finder Reveals A High Mass
and a Low Obliquity for the Young Neptune K2-25b. The Astronomical
Journal, 2020 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200804122225.htm

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