The Kepler Space Telescope has discovered over 4000 candidate
planets, around 40% of which are in systems with multiple planets. Many of the
early multiple planet systems discovered contained one or more giant planets
(planets of similar size to, or larger than, Jupiter), which tend to be easier
to detect. More recent discoveries have found a wider range of planets,
enabling comparisons to be drawn about the systems that host these planets. All
of the giant planets have been found in systems with metal rich stars, indicating
that they are relatively young and have formed late in the Universe’s history
(all metal in the Universe is thought to have formed in old and dying stars,
and as the universe has aged the amount of metal has steadily increased, so
that younger stars contain more metal from the outset), but smaller planets
have been found around a much wider range of stars, implying that such planets
began to form much earlier in the Universe’s 13.8 billion year history.
Kepler 444 was formerly designated as KOI 3158 (Kepler Object of
Interest 3158, implying the 3158th star in the Kepler field found to
have candidate planets), becoming Kepler 444 with the conformation of its
planets (Kepler 444 implying the 444th system in the Kepler field
with confirmed planets). It also has the catalogue designations HIP 94931, KIC
6278762 and LHS 3450. It is a K-type dwarf star, with about 75% of the mass of
the Sun and an effective surface temperature of 5046 K (compared to 5778 K for
our Sun).
It is also one of the closest stars in the Kepler study, at a
distance of only 116.4 light years, and is part of the Arcturus Stream, a group
of ancient stars with similar proper motions; these were formerly thought to
have formed outside the Milky Way proper, in a now defunct satellite galaxy,
though this is now considered unlikely, with the stars probably originating in
the Thick Disk, an area ranging from one to five kiloparsecs of the galactic
plane (85% of the galaxy’s stars originate within the Thin Disk, which lies
within one kiloparsec of the galactic plane).
These observations found a second, dimmer star at a distance of 1.8
arcseconds from Kepler 444 (an arcsecond is one sixtieth of an arcminute, which
is one sixtieth of a degree, which is one three hundred and sixtieth of the sky
envisioned as a sphere surrounding the Earth). This second star shows a common
radial velocity with the primary star, suggesting that it is orbiting Kepler
444 roughly once every 430 years. Data from the HIRES spectograph further
suggests that this second body is itself a pair of M-type Red Dwarf stars, the
larger of which has an effective surface temperature of 3464 K, the smaller
being cooler.
NIRC2 adaptive optics image of Kepler-444. The image
was obtained using the K’filter (2:124mm) for a total of 378 s of integration time. Declination and right
ascension coordinates (J2000.0) are given alongthe vertical and horizontal
axes, respectively. Campante et al.
(2015).
The Kepler 444 system had five planetary candidates, all of which
were resolved to be true planets rather than background objects with a 99.9%
certainty. The planets are all have short orbital periods, implying that they
are all close to the star, within 0.06 AU (i.e. 6% of the distance at which the
Earth orbits the Sun), and the orbital periods appear to be resonant; which is
what would be expected from tightly packed planets.
Next Campante et al.
examined the possibility that the planets might orbit one or both of the
companions, rather than Kepler 444 itself. Since the data from the Kepler Space
Telescope records the amount of dimming that occurs when a planet passes in
front of a star, it is possible to calculate the radius of that planet if the
brightness of the star is known. This suggests that the planets would have radii
of 1.33, 1.64, 1.75, 1.80, and 2.45 times that of the Earth if they are
orbiting the companion bodies. Since it would be virtually impossible for
planets to retain gaseous envelopes or significant volumes of ice this close to
a star, these bodies would have to be rocky in nature, and therefore have
significant masses. Attempts to model such a system with bodies of such high
masses close together suggested that such a system would be unstable and the
planets would be lost within decades or centuries, ruling out this scenario.
This suggests that the five planets are indeed orbiting the K type
star, from which their radii can be calculated to be 0.403, 0.497, 0.530,
0.546, and 0.741 times that of the Earth respectively, a much more stable
configuration.
Thus the first planet, Kepler 444b (when naming bodies in other star
systems planets are given lower case letters and stars upper case letters),
will have a radius 40.3% of the Earth’s (roughly comparable with Mercury), and
orbit the Keppler 444A (the main star in the system) every 3.6 days, at a
distance of 0.04178 AU (4.178% of the average distance between the Earth and
the Sun).
The second planet, Kepler 444c will have a radius 49.7% of that of
the Earth (comparable to Mars), and orbit Kepler 444A every 4.5 days at a
distance of 0.04881 AU.
Kepler 444d will have a radius 53.0% of that of the Earth (again
comparable to that of Mars) and orbit Kepler 444A every 6.2 days at a distance
of 0.06 AU.
Kepler 444e will have a radius 54.6% of that of the Earth, and orbit
Kepler 444A every 7.7 days at a distance of 0.0696 AU.
Finally Kepler 444f will have a radius 74.1% of that of the Earth
(intermediate between those of Mars and Venus) and orbit Kepler 444A every 9.7
days at a distance of 0.0811 AU.
Semi-major axes of planets belonging to the
highly-compact multiple-planet systems Kepler-444, Kepler-11, Kepler-32,
Kepler-33, and Kepler-80. Semi major axes of planets in the Solar System are
shownfor comparison. The vertical dotted line marks the semi-major axis of
Mercury. Symbol size is proportionalto planetary radius. Note that all planets
in the Kepler-444 system are interior to the orbit of the innermostplanet in
the Kepler-11 system, the prototype of this class of highly-compact
multiple-planet systems.Campante et al.
(2015).
Kepler 444 is the oldest star yet found to host planets, the former
holder of which title was Kepler 10, at an age of 10.4 billion years. It is
considered to be a second generation star (i.e. made of matter that had
previously been in only one other star since the origin of the universe),
unlike the Sun and most other known planet-hosting stars, which are third
generation stars. This means that at the time when the Solar System was
forming, the planets of the Kepler 444 system were already older than the Earth
is now. The existence of rocky planets this early in the history of the
Universe suggests that life may also have originated very early, and that truly
ancient ecosystems may exist somewhere in the Universe.
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