An international team including astronomers from
Tokyo Institute of Technology and Aarhus University discovered evidence
that in some cases protoplanetary disks, gas and dust disks
surrounding young stars in which planets form, may have been flipped
over, resulting in backwards or retrograde orbits of the planets. This
finding is an important step in understanding planet formation and
orbital evolution.
In the Solar System, the eight planets orbit
in the same direction, this is also the same direction that the Sun
rotates. Over the last decade researchers discovered that this is not
always the case with exoplanets, planets which orbit around stars other
than the Sun. Some exoplanets have orbits tilted wildly away from the
rotation of the star. Some even have orbits tipped over backwards with
respect to the central star. The mechanisms by which this wide diversity
of orbits come to be is an important question in modern astronomy.
In
one possible mechanism, while the planets are still forming in a
protoplanetary disk of gas and dust around a young star, the gravity of a
nearby companion star can tilt the disk so that the planets form on
misaligned orbits from the beginning. Here, ‘nearby’ means within about a
light-year. This disk-tilt mechanism was first proposed in 2012, but no
confirmed examples had been observed until now.
The exoplanetary
system, K2-290, is an ideal laboratory to test this disk-tilt
mechanism. It has a nearby companion star identified by the Subaru
Telescope (Note 1). It also has two planets with coplanar orbits, orbits
that are aligned with each other. The presence of two planets with
similar orbits is important because it rules out scenarios where
interactions between the planets cause the orbits to tilt.
The
team including Dr. Teruyuki Hirano (Tokyo Institute of
Technology/National Institutes of Natural Sciences) and Dr. Maria Hjorth
(Aarhus University/Tokyo Institute of Technology) used the Subaru
Telescope and other telescopes (Note 2) to measure the misalignment
between the orbits of the planets and the central star. They found that
the planet’s orbits have flipped over nearly backwards. Numerical
simulations by the team showed that the companion would have been able
to flip over the protoplanetary disk, imparting retrograde orbits on the
resulting planets. This provides strong evidence that the disk-tilt
mechanism could be responsible for misaligned orbits, not just in this
case, but in other exoplanet systems as well. It is no longer safe to
assume that planetary orbits are initially aligned with the rotation of
the star just after the formation of the planets in their disk. While
other theories to explain spin-orbit misalignments in exoplanet systems
tend to work best on large, Jupiter-like planets in short period orbits,
the disk-tilt mechanism applies to planets of any size; even
potentially habitable Earth-sized planets.
These results
appeared as Hjorth et. al. "A backward-spinning star with two coplanar
planets" in the Proceedings of the National Academy of Sciences of the
United States of America on February 15, 2021.
References:
Note 1: The companion star was discovered in 2018 using IRCS on the Subaru Telescope. The result appeared as Hjorth et al. "K2-290: a warm Jupiter and a mini-Neptune in a triple-star system" in Monthly Notices of the Royal Astronomical Society on January 15, 2019.
Note
2: The observations were made in 2019 using three high-dispersion
spectrographs: HDS on the Subaru Telescope, HARPS-N on the Telescopio
Nazionale Galileo, and ESPRESSO on the VLT. The alignments between the
two planets' orbits and the star's rotation were measured using the
Rossiter-McLaughlin (RM) effect. See "Inclined Orbits Prevail in Exoplanetary Systems" (press release on January 12, 2011)
for other examples of misaligned exoplanet systems measured with the RM effect.
- A backward-spinning star with two coplanar orbiting planets in a multi stellar system (Aarhus University, February 15, 2021 Press Release)
- Two backward orbiting exoplanets in a triple stellar system prove protoplanetary disk-tilt mechanism (Telescopio Nazionale Galileo, February 15, 2021 Press Release)
Source: Subaru Telescope
