Spotting planets a long way from their stars is no easy proposition when you’re using radial velocity methods. The idea is to track the minute movement of the star as it is affected by an orbiting planet, which shows up as a Doppler shift in the data. What we’re actually seeing is the star and planet orbiting the center of gravity, an indirect method of detection that observes not the planet itself but the effects of the planet as it produces this variation in radial velocity.
The first exoplanets were detected this way, and the method has continued to produce new discoveries. But as a planet’s distance from its star increases, radial velocity becomes tricky to use. Now observation times become extended as the planet completes its longer orbit. We face the same issue with the transit method, which charts the drop in brightness as a planet moves across the face of its star as seen from Earth. Here, too, planets in distant orbits around their star are hard to detect because of the lengthy period of time between individual transits.
Because of these issues, we have little data on the occurrence rate of planets in wide orbits, and that is a problem for analyzing planet formation theories like core accretion, gravitational disk instability and planet migration. The long-term radial velocity data of a host star with a companion object beyond 10 AU shows an almost linear trend over a short observing period. Detection of such a trend is not in itself enough to identify the source of the RV signal.
Image: Periodic change in the radial velocity of the primary star shown left is the strong signature of a planet. When the companion object is far way (farther than 10 AU), the orbital period is so long that the radial velocity shows a linear change, as shown in the right panel. (Credit: NAOJ).
Researchers from the Tokyo Institute of Technology have been trying to solve this problem by combining radial velocity methods with direct imaging. With the latter, the planet detection actually becomes easier as the distance between star and planet widens, because the light of the companion object is not swamped by proximity to the host star. The Tokyo team, using an instrument at Okayama Astrophysical Observatory (NAOJ) has targeted stars 1.5 to 5 times as massive as the Sun, looking not just for long period planets but also companion stars.
A long-term radial velocity trend flags such objects, but it’s the follow-up with direct imaging (in this case using the HiCIAO imager at the Subaru Telescope) that has paid off in an analysis of six intermediate-mass giant stars. All six of these stars showed a long-term radial velocity trend. The question thus becomes, is the source of the trend a companion star or a planet?
HiCIAO is a coronagraph that masks the light of the primary star to allow detection of fainter objects. Three of the observed stars — ? Hydra, HD 5608, and HS 109272 — show companion stars in this analysis, while around three others — ? Draconis, 18 Delphinus, and HD 14067 — companion objects more massive than 60 Jupiter masses can be excluded. The latter are considered candidates for hosting brown dwarfs, while 18 Delphinus is the most likely prospect for hosting a high-mass planet (~ 10-50 AU) that is below the current detection limit.
Image: Objects in the yellow circle were detected by the current study. White circles or round squares show the position of the primary star. The primary was masked during the observation to help the detection of the fainter objects nearby. (Credit: NAOJ)
Combining radial velocity with direct imaging has allowed the team to confirm that the companion objects they found were the cause of the long-term trend in the RV data. On a broader level, these methods may help us understand the distribution of planets around stars more massive than the Sun, as the paper on this work notes:
At Okayama Astrophysical Observatory (OAO), an RV survey targeting intermediate-mass giants (1.5–5 M?) has been conducted for over a decade (e.g., Sato et al. 2003). Sato et al. (2008) found that there is a difference between orbits of planets around intermediate-mass stars and around lower-mass FGK stars. Most planets around intermediate-mass stars have a semi-major axis larger than 0.6 AU, while FGK stars have shorter-period planets. Hence, it was suggested that the orbital distribution of exoplanets around intermediate-mass stars is different from that around solar-type stars.
Is the suggestion valid, or simply the result of our limited datasets? The paper continues:
In addition, the OAO survey detected long-term RV trends in several targets, which indicates the presence of distant companions around them… Identifying the companions that generate the RV trend can improve our knowledge of exoplanet populations for intermediate-mass stars, which are not well understood compared to solar-type stars.
This is not the only study that has combined direct imaging and radial velocity trends. In fact, a project called TRENDS (TaRgetting bENchmark objects with Doppler Spectroscopy), led by Justin Crepp (Notre Dame) has detected three low-mass stellar companions using these methods, along with a white dwarf and a brown dwarf companion, working with data on host stars ranging from F-class down to M-dwarfs. Clearly, direct imaging can be a benefit as we try to work out the source of RV trends that point to the existence of a distant companion.
The paper is Ryu et al., “High-contrast Imaging of Intermediate-mass Giants with Long-term Radial Velocity Trends,” Astrophysical Journal Vol. 825, No. 2 (12 July 2016). Abstract / preprint.
For those interested in a more in-depth discussion of how common Jupiter analogs are, there is my post on Centauri Dreams from earlier this year, “Where Are the Jupiter Analogs?”
https://centauri-dreams.org/?p=34667
Well, I never read the full mission summary on the Kepler Mission.
It was supposed to find Terretrials, but as bonus it found these
large Jovians beyond the orbit of 1.0 AU.
Kepler 167 e. at 1.89 AU
Kepler 68d at 1.4 AU
Kepler 419c at 1.68 AU
So the question has to be, IF Kepler had functioned for 7 years. could
it have found More Candidate Jovians beyond 1.89 AU, around K type stars. (in terms of similar solar radiation I think this would be at 2.0-3.0 AU distant as a SWAG. Or is the probability of a transit just too low because of the distance from the earth ( Is it a good rule thumb to assume the size of the larger planet is cancelled out the smaller size of the Type K sun for detection probability factors)
Yes, if Kepler had been able to continue to monitor its original primary mission star field longer, it would have been able to detect Jupiter-size planets in larger orbits with longer periods. Of course the odds that such a planet has an orbit aligned to produce a transit decreases linearly with increasing orbital radius (i.e. a planet in a 2 AU orbit is half as likely to produce a transit as a planet in a 1 AU orbit). But in order to confirm the existence of a planet in a 2.0 to 3.0 AU orbit around a Sun-like star, 11 to 21 years of data would be needed to ensure three transits were observed (the minimum “standard” adopted for the Kepler mission – two transits to pin down the period and a third to confirm).
Whatever happened to measuring the position of the star as it “wobbles” while moving among background stars? Astronomers were trying this method long before they tried radial velocity and transit measurements (60s and 70s?). It seems like this method would be easier to use with a planet in a far orbit than a near one, since the center of mass of the star and planet will be farther away from the center of the star. With enough accuracy it should be possible to plot the shape of the wobble path to determine the angle at which the orbit is tilted to the line of sight, and thus get a more accurate estimate of the planet’s mass, instead of just the minimum mass that is provided by the radial velocity method.
Using astrometry (the precision measurement of stars’ positions and how they change over time) to find extrasolar planets is certainly possible and has been employed (unsuccessfully, so far) for over a half a century. The most (in)famous target for using this method has got to be the nearby red dwarf star, Barnard’s Star which was thought at one time to harbor two Jupiter-size exoplanets (which are no known not to exist):
http://www.drewexmachina.com/2015/04/23/search-for-planets/
It has also been known for a loooong time that astrometric searches for exoplanets complement those performed using the radial velocity technique (the former is good at finding large planets in more distant orbits while the latter is ideal for finding those in smaller orbits). The problem has been getting enough astrometric measurements with sufficient precision over a long enough period of time to actually detect something. ESA’s Gaia mission currently taking measurements from the Sun-Earth L2 point (which just had its first data release for the first 14 months of its nominal five-year primary mission) is expected to be able to be sensitive enough and operate long enough to detect hundreds or even thousands of exoplanets astrometrically. We are just a few years away from having the data needed to make these detections.
There is another new method to find exoplanets using Stellar Seismology and reflection from exoplanets. This method uses the reflection of the oscillation from the star and filters it out to see the exoplanet . I have been trying to find the original article but have not been able to find it again on the internet so if someone has the info I would greatly appreciate the address.
Can’t find anything either, sorry.
I’m assuming this is different to using the Phase of the exoplanets? which isn’t new and there’s a ton of stuff on google.
Does this technique you mention rely on the time delay for the reflections (ie ET would detect a small, 8-minute-delayed signal from the earth reflecting the sun’s signal) or am I way off?
Astronomers find a planet through a never-before-used method
They used pulsation to confirm a long-period planet around a Kepler candidate world.
By Korey Haynes | Published: Tuesday, October 04, 2016
Astronomers find most exoplanets from indirect signals, noticing changes in the light of the planet’s host star instead of by seeing the planet itself. But some stars’ light changes all on its own, making these methods tricky at best. KIC 7917485b is the first exoplanet identified around a main sequence A-type star from its orbital motion, and the first found near an A -typestar’s habitable zone.
A-type stars are bigger and hotter than most stars in the Kepler catalog and tend to be noisy, changing brightness at regular intervals. This dimming and brightening can be hard to untangle from, for instance, a planet transiting and dimming its light. As such, while there’s no reason for A-type stars not to have planets, it’s been difficult for astronomers to identify them. So far, the few exoplanets found around A-type stars are either from direct imaging (which can only, where the planets are very far from their star, or from transits where the planets are very close to the star, where the signal is strong.
But astronomers came up with a novel idea to use the variability of the star itself as a way to look for exoplanets. The star pulses because of helium changes in its lower layers. It puffs up, cools and dims, shrinks, heats and brightens, and then repeats the process multiple times in a day. In a Kepler light curve, this shows up as a periodic dimming and brightening, like clockwork. But this clock shows a delay. The pulsations appear a little early or late, and by calculating this delay, astronomers can measure that the star is actually moving in a back-and-forth, orbital motion. And this movement is due to the gravitational tug of a nearby planet.
Full article here:
http://www.astronomy.com/news/2016/10/astronomers-find-a-planet-through-a-never-before-used-method
To quote:
The delays in KIC 7917485’s pulsations revealed a planet about 12 Jupiter masses with a period of 840 days, which is close to the habitable zone of such a hot star. While 12 Jupiter masses makes this planet nearly a brown dwarf, and certainly a gas giant, the study’s authors point out that potential moons keep the question of habitability an intriguing one.
The pulsation delays are very similar to how astronomers find planets via the radial velocity method, but in this case, no spectrometer is needed. The Kepler light curve provides all the necessary information; the planet doesn’t need to transit to reveal itself. This is key, because most planets on orbits that take hundreds of days— planets in the habitable zone of hot stars — won’t. Having a method that can reveal them anyway is an important tool in the exoplanet-finding kit.
Probably apropos of nothing, but I want to give this a shot.
Could it be that the orbits of planets influence their star’s peculiar motion? Might be similar to an out-of-balance washing machine walking across the floor, since orbital systems are like unbalanced rotational bodies?
First Pluto, now this: Discovery of first binary-binary calls solar system formation into question
October 19, 2016
Rachel Wayne
Everything we know about the formation of solar systems might be wrong, says University of Florida astronomy professor Jian Ge and his postdoc, Bo Ma. They’ve discovered the first “binary–binary” – two massive companions around one star in a close binary system, one so-called giant planet and one brown dwarf, or “failed star” The first, called MARVELS-7a, is 12 times the mass of Jupiter, while the second, MARVELS-7b, has 57 times the mass of Jupiter.
Astronomers believe that planets in our solar system formed from a collapsed disk-like gaseous cloud, with our largest planet, Jupiter, buffered from smaller planets by the asteroid belt. In the new binary system, HD 87646, the two giant companions are close to the minimum mass for burning deuterium and hydrogen, meaning that they have accumulated far more dust and gas than what a typical collapsed disk-like gaseous cloud can provide. They were likely formed through another mechanism. The stability of the system despite such massive bodies in close proximity raises new questions about how protoplanetary disks form. The findings, which are now online, will be published in the November issue of the Astronomical Journal.
https://arxiv.org/abs/1608.03597
Full article here:
http://news.ufl.edu/articles/2016/10/first-pluto-now-this-discovery-of-first-binary-binary-calls-solar-system-formation-into-question.php