NEWS NOTES
10 AUGUST 2019 • SKY & TELESCOPE
ASTRONOMERS OBSERVING NEARBY
interstellar space have fi nally detected
helium hydride (HeH+), the fi rst mol-
ecule to form in the early universe. Rolf
Güsten (Max Planck Institute for Radio
Astronomy, Germany) and colleagues
report the fi nd in the April 18th Nature.
The early universe contained mostly
hydrogen and helium, so it’s perhaps
unsurprising that its fi rst molecule
would be helium hydride. But of all the
elements, helium requires the most
energy to remove an electron — it’s no
easy feat for it to combine with another
atom. Indeed, the ion didn’t survive
long even in the unique conditions of
the early universe, but it played a brief
but important role. Its destruction gave
rise to molecular hydrogen (H 2 ), which
eventually permeated galaxies and
star-forming reservoirs. The existence of
HeH+ is thus instrumental to under-
standing early chemical evolution.
Yet for years, astronomers tried —
and failed — to detect the molecule.
Although it was created in a lab in 1925,
its fragility makes it rare in the natural
world. Astronomers in the 1970s real-
ized they might fi nd the molecule newly
created in nearby plasmas, but a defi ni-
tive detection eluded them.
So Güsten and colleagues took to the
skies. They fl ew with the Stratospheric
Observatory for Infrared Astronomy
aboard a modifi ed Boeing 747, soar-
ing above the infrared-absorbing lower
atmosphere (S&T: Apr. 2015, p. 60)
to observe a planetary nebula named
NGC 7027. The nebula is an ideal target
because the blazingly hot 600-year-old
white dwarf at its center emits ultra-
violet radiation that strips electrons off
hydrogen and helium atoms alike. The
spherical region where helium is ion-
ized is slightly larger than where both
hydrogen and helium are ionized; in
this outer region, ionized helium is able
to bond with neutral hydrogen to make
helium hydride in a thin shell around
the star. Using the German Receiver
for Astronomy at Terahertz Frequen-
cies instrument, Güsten and colleaguesdetected radiation from the ion at
149.137 microns.
Having fi nally discovered HeH+, the
team also studied the formation and
destruction mechanisms of the mole-
cule that kick-started cosmic chemistry.
■ MONICA YOUNG
u Astronomers found helium hydride mol-
ecules (inset) in the heated gas of the planetary
nebula NGC 7027.pLeft: A new image of LkCa 15 reveals two arcs of light. Middle: A theoretical model with an inner
and outer disk reproduces the arcs. Right: Another theoretical model shows what the image would
look like if the innermost arc were actually multiple planets.EXCEPTIONAL NEW IMAGES of the LkCa 15 system, a young, Sun-like star once
thought to host multiple gas giant protoplanets, show that these particular planets
probably don’t exist after all.
LkCa 15 has excited astronomers ever since interferometry and direct observa-
tions of hydrogen gas fl ows turned up evidence of up to three gas giants being born
in the protoplanetary disk of gas and dust around the star (S&T: Aug. 2012, p. 20).
These putative planets orbit inside the large outer disk, having presumably cleared
out the inner region. Since LkCa 15 is only a couple million years old, the same
age as our Sun was when Jupiter and Saturn took shape, the discovery promised
insights into the birth of a solar system.
But Thayne Currie (NASA Ames Research Center) and colleagues are throwing
those claims into question with a new study to be published in the Astrophysical
Journal Letters. The team used the Subaru Coronagraphic Extreme Adaptive Optics
system at the Subaru Telescope to take a sharper image of the LkCa 15 system than
ever before obtained.
The resulting view doesn’t show any baby Jupiters; instead, it shows an inner
disk in addition to the outer one. Starlight scattered off the inner disk has the same
brightness as the signals previously attributed to protoplanets.
Keck Observatory images taken in 2009 and 2017 confi rm that this bright arc
remained in the same place over the years. If the light were coming from unre-
solved planets, the arc would have rotated around the star.
Nevertheless, Kate Follette (Amherst College), who was not involved in the new
study, thinks the data still support the existence of at least one protoplanet, and
Currie agrees: “I think it’s very clear that there are planets around the star. They’re
just fainter than we previously thought.” Distinguishing infant planets from the
disk they’re born in will continue to be a challenge.
■ STEVEN MURRAYCOSMOLOGY
Astronomers Find Universe’s First Type of MoleculeEXOPLANETS
Solar System “Twin” Is Missing Infant JupitersPL
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KCA^15 :^
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