by Paul Gilster | Oct 9, 2018 | Missions |
I want to talk about the Voyagers this morning and their continuing interstellar mission, but first, a quick correction. Yesterday in writing about New Horizons’ flyby of MU69, I made an inexplicable gaffe, referring to the event as occurring on the 19th rather than the 1st of January (without my morning coffee, I had evidently fixated on the ’19’ of 2019). Several readers quickly spotted this in the article’s penultimate paragraph and I fixed it, but unfortunately the email subscribers received the uncorrected version. So for the record, we can look forward to the New Horizons flyby of MU69 on January 1, 2019 at 0533 UTC. Sorry about the error.
Let’s turn now to the Voyagers, and the question of how long they will stay alive. I often see 2025 cited as a possible terminus, with each spacecraft capable of communication with Earth and the operation of at least one instrument until then. If we make it to 2025, then Voyager 1 would be 160 AU out, and Voyager 2 will have reached 135 AU or thereabouts. In his book The Interstellar Age, Jim Bell — who worked as an intern on the Voyager science support team at JPL starting in 1980, with Voyager at Saturn — notes that cycling off some of the remaining instruments after 2020 could push the date further, maybe to the late 2020s.
After that? With steadily decreasing power levels, some heaters and engineering subsystems will have to be shut down, and with them the science instruments, starting with the most power-hungry. Low-power instruments like the magnetometer could likely stay on longer.
And then there’s this possibility. Stretching out their lifetimes might demand reducing the Voyagers’ output to an engineering signal and nothing else. Bell quotes Voyager project scientist Ed Stone: “As long as we have a few watts left, we’ll try to measure something.”
Working with nothing more than this faint signal, some science could be done simply by monitoring it over the years as it recedes. Keep Voyager doing science until 2027 and we will have achieved fifty years of science returns. Reduced to that single engineering signal, the Voyagers might stay in radio contact until the 2030s.
Image: This graphic shows the position of the Voyager 1 and Voyager 2 probes, relative to the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in 2012. Voyager 2 is still in the heliosheath, or the outermost part of the heliosphere. Credit: NASA/JPL-Caltech.
Bear in mind the conditions the Voyagers have to deal with even now. With more than 25 percent of their plutonium having decayed, power limitations are a real factor. If either Voyager passed by an interesting inner Oort object, the cameras could not be turned on because of the power drain, which would shut down their heaters. Continuing power demands from radio communications and the heaters create thorny problems for controllers.
Even so, we’re still doing good science. Today the focus is on a still active Voyager 2, which may be nearing interstellar space. Voyager 2 is now 17.7 billion kilometers from Earth, about 118 AU, and has been traveling through the outermost layers of the heliosphere since 2007. The solar wind dominates this malleable region, which changes during the Sun’s eleven year activity cycle. Solar flares and coronal mass ejections all factor into its size and shape. Ahead is the heliosphere’s outer boundary, the heliopause, beyond which lies interstellar space.
The Cosmic Ray Subsystem instrument on Voyager 2 is now picking up a 5 percent increase in the rate of cosmic rays hitting the spacecraft as compared with what we saw in early August. Moreover, the Low-Energy Charged Particle instrument is picking up an increase in higher-energy cosmic rays. The increases parallel what Voyager 1 found beginning in May, 2012, about three months before it exited the heliopause.
So we may be about to get another interstellar spacecraft. Says Ed Stone:
“We’re seeing a change in the environment around Voyager 2, there’s no doubt about that. We’re going to learn a lot in the coming months, but we still don’t know when we’ll reach the heliopause. We’re not there yet — that’s one thing I can say with confidence.”
We haven’t crossed the outer regions of the heliosphere with a functioning spacecraft more than once, so there is a lot to learn. Voyager 2 moves through a different part of the outer heliosphere — the heliosheath — than Voyager 1 did, so we can’t project too much into the timeline. We’ll simply have to keep monitoring the craft to see what happens.
What an extraordinary ride it has been, and here’s hoping we can keep both Voyagers alive as long as possible. Even when their power is definitively gone, they’ll still be inspiring our imaginations. We’re 300 years from the Inner Oort Cloud, and a whopping 30,000 years from the Oort Cloud’s outer edge. In 40,000 years, Voyager 2 will pass about 110,000 AU from the red dwarf Ross 248, which will at that time be the closest star to the Sun. Both spacecraft will eventually follow 250-million year orbits around the center of the Milky Way.
I return to Jim Bell, who waxes poetic at the thought. He envisions a far future when our remote descendants may be able to see the Voyagers again. Here is a breathtaking vision indeed:
Over time — enormous spans of time, as the gravity of passing stars and interacting galaxies jostles them as well as the stars in our galaxy — I imagine that the Voyagers will slowly rise out of the plane of our Milky Way, rising, rising ever higher above the surrounding disk of stars and gas and dust, as they once rose above the plane of their home stellar system. If our far-distant descendants remember them, then our patience, perseverance, and persistence could be rewarded with perspective when our species — whatever it has become — does, ultimately, follow them. The Voyagers will be long dormant when we catch them, but they will once again make our spirits soar as we gaze upon these most ancient of human artifacts, and then turn around and look back. I have no idea if they’ll still call it a selfie then, but regardless of what it’s called, the view of our home galaxy, from the outside, will be glorious to behold.
by Paul Gilster | Oct 8, 2018 | Outer Solar System |
I love the timing of New Horizons’ next encounter, just as we begin a new year in 2019. On the one hand, we’ll be able to look back to a mission that has proven successful in some ways beyond the dreams of its creators. On the other hand, we’ll have the first close-up brush past a Kuiper Belt Object, 2014 MU69 or, as it’s now nicknamed, Ultima Thule. This farthest Solar System object ever visited by a spacecraft may, in turn, be followed by yet another still farther, if all goes well and the mission is extended. This assumes, of course, another target in range.
We can’t rule out a healthy future for this spacecraft after Ultima Thule. Bear in mind that New Horizons seems to be approaching its current target along its rotational axis. That could reduce the need for additional maneuvers to improve visibility for the New Horizons cameras, saving fuel for later trajectory changes if indeed another target can be found. The current mission extension ends in 2021, but another extension would get a powerful boost if new facilities like the Large Synoptic Survey Telescope become available, offering more capability at tracking down an appropriate KBO. Hubble and New Horizons itself will also keep looking.
But even lacking such a secondary target, an operational New Horizons could return useful data about conditions in the outer Solar System and the heliosphere, with the spacecraft’s radioisotope thermoelectric generator still producing sufficient power for some years. I’ve seen a worst-case 2026 as the cutoff point, but Alan Stern is on record as saying that the craft has enough hydrazine fuel and power from its plutonium generator to stay functional until 2035.
By way of comparison with Voyager, which we need to revisit tomorrow, New Horizons won’t reach 100 AU until 2038, nicely placed to explore the heliosphere if still operational.
But back to Ultima Thule, a destination now within 112 million kilometers of the spacecraft. New Horizons is closing at a rate of 14.4 kilometers per second, enroute to what the New Horizons team says will need to be a 120 by 320 kilometer ‘box’ in a flyby that needs to be predicted within 140 seconds. Based on what we saw at Pluto/Charon, these demands can be met.
Image: At left, a composite optical navigation image, produced by combining 20 images from the New Horizons Long Range Reconnaissance Imager (LORRI) acquired on Sept. 24. The center photo is a composite optical navigation image of Ultima Thule after subtracting the background star field; star field subtraction is an important component of optical navigation image processing since it isolates Ultima from nearby stars. At right is a magnified view of the star-subtracted image, showing the close proximity and relative agreement between the observed and predicted locations of Ultima. Credit: NASA/JHUAPL/SwRI/KinetX.
Above are the latest navigation images from New Horizons’ Long Range Reconnaissance Imager (LORRI). An engine burn on October 3 further tightened location and timing information for the New Year’s flyby, a 3 ½ minute maneuver that adjusted the spacecraft’s trajectory and increased its speed by 2.1 meters per second. Records fall every time New Horizons does this, with the October 3 correction marking the farthest course correction ever performed.
It’s interesting to learn, too, that this is the first time New Horizons has made a targeting maneuver for the Ultima Thule flyby that used pictures taken by New Horizons itself. The ‘aim point’ is 3,500 kilometers from Ultima at closest approach, and we’ve just learned that these navigation images confirm that Ultima is within 500 kilometers of its expected position.
“Since we are flying very fast and close to the surface of Ultima, approximately four times closer than the Pluto flyby in July 2015, the timing of the flyby must be very accurate,” said Derek Nelson, of KinetX Aerospace, Inc., New Horizons optical navigation lead. “The images help to determine the position and timing of the flyby, but we must also trust the prior estimate of Ultima’s position and velocity to ensure a successful flyby. These first images give us confidence that Ultima is where we expected it to be, and the timing of the flyby will be accurate.”
I’m already imagining New Year’s eve with Ultima Thule to look forward to. You can adjust your own plans depending on your time zone, but the projected flyby time is 0533 UTC on the 1st. As with Pluto/Charon, the excitement of the encounter continues to build. In the broader picture, the more good science we do in space, the more drama we produce as we open up new terrain. This week alone, we need to look at the Hayabusa2 operations at Ryugu, the upcoming OSIRIS-REx exploration of asteroid Bennu, and the continuing saga of Voyager 2.
But New Horizons also reminds us of an uncomfortable fact. When it comes to the outer system, this is the only spacecraft making studies of the Kuiper Belt from within it, and there is no other currently planned. Data from this mission will need to carry us for quite some time.
by Paul Gilster | Oct 5, 2018 | Missions |
Last Monday’s article on the Trillion Planet Survey led to an email conversation with Phil Lubin, its founder, in which the topic of Breakthrough Starshot invariably came up. When I’ve spoken to Dr. Lubin before, it’s been at meetings related to Starshot or presentations on his DE-STAR concept. Standing for Directed Energy System for Targeting of Asteroids and exploRation, DE-STAR is a phased laser array that could drive a small payload to high velocities. We’ve often looked in these pages at the rich history of beamed propulsion, but how did the DE-STAR concept evolve in Lubin’s work for NASA’s Innovative Advanced Concepts office, and what was the path that led it to the Breakthrough Starshot team?
The timeline below gives the answer, and it’s timely because a number of readers have asked me about this connection. Dr. Lubin is a professor of physics at UC-Santa Barbara whose primary research beyond DE-STAR has involved the early universe in millimeter wavelength bands, and a co-investigator on the Planck mission with more than 400 papers to his credit. He is co-recipient of the 2006 Gruber Prize in Cosmology along with the COBE science team for their groundbreaking work in cosmology. Below, he tells us how DE-STAR emerged.
By Philip Lubin
June 2009: Philip Lubin begins work on large scale directed energy systems at UC Santa Barbara. Baseline developed is laser phased array using MOPA [master oscillator power amplifier, a configuration consisting of a master laser (or seed laser) and an optical amplifier to boost the output power] topology. The DE system using this topology is named DE-STAR (Directed Energy System for Targeting Asteroids and exploRation). Initial focus is on planetary defense and relativistic propulsion. Development program begins. More than 250 students involved in DE R&D at UCSB since.
February 14, 2013: UCSB group has press release about DE-STAR program to generate public discussion about applications of DE to planetary defense in anticipation of February 15 asteroid 2012 DA14, which was to come within geosync orbit. On February 15 Chelyabinsk meteor/asteroid hit. This singular coincidence of press release and hit the next day generated a significant change in interest in possible use of large scale DE for space applications. This “pushed the DE ball over the hill”.
August 2013: Philip Lubin and group begin publication of detailed technical papers in multiple journals. DE-STAR program is introduced at invited SPIE [Society of Photo-optical Instrumentation Engineers] plenary talk in San Diego at Annual Photonics meeting. More than 50 technical papers and nearly 100 colloquia from his group have emerged since then. List of DE-STAR papers can be found here.
August 2013: 1st Interstellar Congress held in Dallas, Texas by Icarus Interstellar. Eric Malroy introduces concepts for the use of nanomaterials in sails.
August 2013: First proposal submitted to NASA for DE-STAR system from UC Santa Barbara.
January 2014: Work begins on extending previous UCSB paper to much longer “roadmap” paper which becomes “A Roadmap to Interstellar Flight” (see below).
February 11, 2014 – Lubin gives colloquium on DE-STAR at the SETI Institute in Mountain View, CA. Summarizes UCSB DE program for planetary defense, relativistic propulsion and implications for SETI. SETI Institute researchers suggest Lubin speak with NASA Ames director Pete Worden as he was not at the talk. Worden eventually leaves Ames a year later on March 31, 2015 to go to the Breakthrough Foundation. Lubin and Worden do not meet until 18 months later at the Santa Clara 100YSS meeting (see below).
August 2014: Second proposal submitted to NASA for DE-STAR driven relativistic spacecraft. Known as DEEP-IN (Directed Energy Propulsion for Interstellar Exploration). Accepted and funded by NASA NIAC program as Phase I program. Program includes directed energy phased array driving wafer scale spacecraft as one option [Phase 1 report “A Roadmap to Interstellar Flight” available here].
April 2015: Lubin submits the “roadmap” paper to the Journal of the British Interplanetary Society.
June 2015: Lubin presents DE driven relativistic flight at Caltech Keck Institute meeting. Meets with Emmett and Glady W Technology Fund.
August 31, 2015: August 31, 2015: Lubin and Pete Worden attend 100YSS (100 Year Star Ship) conference in Santa Clara, CA [Worden is now executive director, Breakthrough Starshot, and former director of NASA Ames Research Center]. Lubin is invited by Mae Jemison (director of 100YSS) to give a talk about the UCSB NASA DE program as a viable path to interstellar flight. Worden has to leave before Lubin’s talk, but in a hallway meeting Lubin informs Worden of the UCSB NASA Phase I NASA program for DE driven relativistic flight. This meeting takes places as Lubin recalls Feb 2014 SETI meeting where a discussion with Worden is suggested. Worden asks for further information about the NASA program and Lubin sends Worden the paper “A Roadmap to Interstellar Flight” summarizing the NASA DEEP-IN program. Worden subsequently forwards paper to Yuri Milner.
December 16, 2015: Lubin, Worden and Pete Klupar [chief engineer at Breakthrough Prize Foundation] meet at NASA Ames to discuss DEEP-IN program and “roadmap” paper.
December 2015: Milner calls for meeting with Lubin to discuss DEEP-IN program, “roadmap” paper and the prospects for relativistic flight.
January 2016: Private sector funding of UCSB DE for relativistic flight effort by Emmett and Glady W Technology Fund begins. Unknown to public – anonymous investor greatly enhances UCSB DE effort.
January 2016: First meeting with Milner in Palo Alto. Present are Lubin, Milner, Avi Loeb (Harvard University), Worden and Klupar. Milner sends “roadmap” paper to be reviewed by other physicists. A long series of calls and meetings ensue. This begins the birth of Breakthrough Starshot program.
March 2016 – NASA Phase II proposal for DEEP-IN submitted. Renamed Starlight subsequently. Accepted and funded by NASA.
March 2016: After multiple reviews of Lubin “roadmap” paper by independent scientists, Breakthrough Initiatives endorses idea of DE driven relativistic flight.
April 12, 2016: Public release of Breakthrough Starshot. Hawking endorses idea at NY public announcement.
To keep up with developments, the following websites are useful:
NASA Starlight (DE-STAR for interstellar relativistic flight):
http://www.deepspace.ucsb.edu/projects/starlight
Planetary Defense Application of DE-STAR:
http://www.deepspace.ucsb.edu/projects/directed-energy-planetary-defense
Implications for SETI:
http://www.deepspace.ucsb.edu/projects/implications-of-directed-energy-for-seti
by Paul Gilster | Oct 4, 2018 | Outer Solar System |
Whether or not there is an undiscovered planet lurking in the farthest reaches of the Solar System, the search for unknown dwarf planets and other objects continues. Extreme Trans-Neptunian objects (ETNOs) are of particular interest. The closest they come to the Sun is well beyond the orbit of Neptune, with the result that they have little gravitational interaction with the giant planets. Consider them as gravitational probes of what lies beyond the Kuiper Belt.
Among the population of ETNOs are the most distant subclass, known as Inner Oort Cloud objects (IOCs), of which we now have three. Added to Sedna and 2012 VP113 comes 2015 TG387, discovered by Scott Sheppard (Carnegie Institution for Science), Chad Trujillo (Northern Arizona University) and David Tholen (University of Hawai?i). The object was first observed in 2015, leading to several years of follow-up observations necessary to obtain a good orbital fit.
For 2015 TG387 is a challenging catch, discovered at about 80 AU from the Sun but normally at far greater distance:
“We think there could be thousands of small bodies like 2015 TG387 out on the Solar System’s fringes, but their distance makes finding them very difficult,” Tholen said. “Currently we would only detect 2015 TG387 when it is near its closest approach to the sun. For some 99 percent of its 40,000-year orbit, it would be too faint to see, even with today’s largest telescopes.”
Image: The orbits of the new extreme dwarf planet 2015 TG387 and its fellow Inner Oort Cloud objects 2012 VP113 and Sedna, as compared with the rest of the Solar System. 2015 TG387 was nicknamed “The Goblin” by its discoverers, since its provisional designation contains “TG”, and the object was first seen around Halloween. Its orbit has a larger semi-major axis than both 2012 VP11 and Sedna, so it travels much farther from the Sun, out to 2300 AU. Credit: Roberto Molar Candanosa and Scott Sheppard / Carnegie Institution for Science.
Perihelion, the closest distance this object gets to the Sun, is now calculated at roughly 65 AU, so we are dealing with an extremely elongated orbit. 2015 TG387 has, after VP113 and Sedna (80 and 76 AU respectively), the third most distant perihelion known, but it has a larger orbital semi-major axis, so its orbit carries it much further from the Sun than either, out to about 2,300 AU. At these distances, Inner Oort Cloud objects are all but isolated from the bulk of the Solar System’s mass.
You may recall that it was Sheppard and Trujillo who discovered 2012 VP113 as well, triggering a flurry of investigation into the orbits of such worlds. The gravitational story is made clear by the fact that Sedna, 2012 VP113 and 2015 TG387 all approach perihelion in the same part of the sky, as do most known Extreme Trans-Neptunian objects, an indication that their orbits are being shaped by something in the outer system. Thus the continuing interest in so-called Planet X, a hypothetical world whose possible orbits were recently modeled by Trujillo and Nathan Kaib (University of Oklahoma).
The simulations show the effect of different Planet X orbits on Extreme Trans-Neptunian objects. In 2016, drawing on previous work from Sheppard and Trujillo, Konstantin Batygin and Michael Brown examined the orbital constraints for a super-Earth at several hundred AU from the Sun in an elliptical orbit. Including such a world in their simulations, the latter duo were able to show that several presumed planetary orbits could result in stable orbits for other Extreme Trans-Neptunian objects. Let’s go to the paper to see how 2015 TG387 fits into the picture:
…Trujillo (2018) ran thousands of simulations of a possible distant planet using the orbital constraints put on this planet by Batygin and Brown (2016a). The simulations varied the orbital parameters of the planet to identify orbits where known ETNOs were most stable. Trujillo (2018) found several planet orbits that would keep most of the ETNOs stable for the age of the solar system.
So we’ve fit the simulated orbit with Sedna, 2012 VP113 and other ETNOs. The next step was obvious:
To see if 2015 TG387 would also be stable to a distant planet when the other ETNOs are stable, we used several of the best planet parameters found by Trujillo (2018). In most simulations involving a distant planet, we found 2015 TG387 is stable for the age of the solar system when the other ETNOs are stable. This is further evidence the planet exists, as 2015 TG387 was not used in the original Trujillo (2018) analysis, but appears to behave similarly as the other ETNOs to a possible very distant massive planet on an eccentric orbit.
Image: Movie of the discovery images of 2015 TG387. Two images were taken about 3 hours apart on October 13, 2015 at the Subaru Telescope on Maunakea, Hawai?i. 2015 TG387 can be seen moving between the images near the center, while the more distant background stars and galaxies remain stationary. Credits: Dave Tholen, Chad Trujillo, Scott Sheppard.
We know very little about 2015 TG387 itself, though the paper, assuming a moderate albedo, finds a likely diameter in the range of 300 kilometers. The stability of this small object’s orbit, keeping it aligned and stable in relation to the eccentric orbit of the hypothesized Planet X, supports the existence of the planet, especially since the derived orbit of 2015 TG387 was determined after the Planet X orbital simulations. Despite this, notes the paper in conclusion, “…2015 TG387 reacts with the planet very similarly to the other known IOCs and ETNOs.”
Another interesting bit: There is a suggestion that ETNOs in retrograde orbit are stable. Given this, the authors do not rule out the idea that the planet itself might be on a retrograde orbit.
The paper is Sheppard et al., “A New High Perihelion Inner Oort Cloud Object,” submitted to The Astronomical Journal (preprint).
by Paul Gilster | Oct 3, 2018 | Exoplanetary Science |
8,000 light years from Earth in the constellation Cygnus, the star designated Kepler 1625 may be harboring a planet with a moon. The planet, Kepler 1625b, is a gas giant several times the mass of Jupiter. What David Kipping (Columbia University) and graduate student Alex Teachey have found is compelling though not definitive evidence of a moon orbiting the confirmed planet.
If we do indeed have a moon here, and upcoming work should be able to resolve the question, we are dealing, at least in part, with the intriguing scenario many scientists (and science fiction writers) have speculated about. Although a gas giant, Kepler 1625b orbits close to or within the habitable zone of its star. A large, rocky moon around it could be a venue for life, but the moon posited for this planet doesn’t qualify. It’s quite large — roughly the size of Neptune — and like its putative parent, a gaseous body. If we can confirm the first exomoon, we’ll have made a major advance, but the quest for habitable exomoons does not begin around Kepler 1625b.
Image: Columbia’s Alex Teachey, lead author of the paper on the detection of a potential exomoon. Credit: Columbia University.
None of this should take away from the importance of the detection, for exploring moons around exoplanets will doubtless teach us a great deal about how such moons form. Unlike the Earth-Moon system, or the Pluto/Charon binary in our own Solar System, Kepler-1625b’s candidate moon would not have formed through a collision between two rocky bodies early in the history of planetary development. We’d like to learn how it got there, if indeed it is there. Far larger than any Solar System moon, it is estimated to be but 1.5% of its companion’s mass.
The methods David Kipping has long espoused through the Hunt for Exomoons with Kepler (HEK) project have come to fruition here. Working with data on 284 planets identified through the Kepler mission, each of them with orbital periods greater than 30 days, Kipping and Teachey found interesting anomalies at Kepler-1625b, where Kepler had recorded three transits. The lightcurves produced by these transits across the face of the star as seen from the spacecraft showed deviations that demanded explanation.
Image: Exomoon hunter David Kipping. Credit: Columbia University.
Their interest heightened, the researchers requested and were awarded 40 hours of time on the Hubble Space Telescope, whose larger aperture could produce data four times more precise than that available from Kepler. On October 28 and 29 of 2017, the scientists took data through 26 Hubble orbits. Examining the lightcurve of a 19-hour long transit of Kepler-1625b, they noted a second, much smaller decrease in the star’s light some 3.5 hours later.
Kipping refers to it as being consistent with “…a moon trailing the planet like a dog following its owner on a leash,” but adds that the Hubble observation window closed before the complete transit of the candidate moon could be measured. The paper addresses this second dip:
The most compelling piece of evidence for an exomoon would be an exomoon transit, in addition to the observed TTV [transit timing variation]. If Kepler-1625b’s early transit were indeed due to an exomoon, then we should expect the moon to transit late on the opposite side of the barycenter. The previously mentioned existence of an apparent flux decrease toward the end of our observations is therefore where we would expect it to be under this hypothesis. Although we have established that this dip is most likely astrophysical, we have yet to discuss its significance or its compatibility with a self-consistent moon model.
In and of itself, this is exciting information, but as noted above, we also learn in this morning’s paper in Science Advances that transit timing variations are apparent here. The planet itself began its transit 77.8 minutes earlier than predicted. One way to account for this is by the pull of a moon on the planet, resulting in their both orbiting a common center of gravity and thus throwing the transit calculation (based on an unaccompanied planet) off. What Kipping and Teachey will need to eliminate is the possibility that a second planet, yet undetected, could have caused the timing variation. There is thus far no evidence from Kepler of such a planet.
“A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation,” said lead author Teachey. “It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation.”
Image: This is Figure 4 from the paper. Caption: Moon solutions. The three transits in Kepler (top) and the October 2017 transit observed with HST (bottom) for the three trend model solutions. The three colored lines show the corresponding trend model solutions for model M, our favored transit model. The shape of the HST transit differs from that of the Kepler transits owing to limb darkening differences between the bandpasses. Credit: David Kipping, Alex Teachey.
One problem with exomoon hunting is that the ideal candidate planets are those in wide orbits, but this makes for long periods between transits. Even so, the number of large planets in orbits farther from their star than 1 AU is growing, and such worlds should be useful targets for the upcoming James Webb Space Telescope. Although we still have to confirm Kepler-1625b’s moon, such a confirmation could prove only the beginning of a growing exomoon census.
We’ll know more as we make more detections, but for now, I think Kipping and Teachey’s caution is commendable. Noting that confirmation will involve long scrutiny, observations and skepticism from within the community, they point out:
…it is difficult to assign a precise probability to the reality of Kepler-1625b-i. Formally, the preference for the moon model over the planet-only model is very high, with a Bayes factor exceeding 400,000. On the other hand, this is a complicated and involved analysis where a minor effect unaccounted for, or an anomalous artifact, could potentially change our interpretation. In short, it is the unknown unknowns that we cannot quantify. These reservations exist because this would be a first-of-its-kind detection — the first exomoon.
A final thought: The paper points out that the original Kepler data that flagged Kepler-1625b as interesting in exomoon terms were actually stronger than the Kepler data Kipping and Teachey added into the mix for this work. They are now working with the most recent release, which had to be revisited for all factors that could affect the analysis. It turns out that this most recent release “only modestly favors that hypothesis when treated in isolation.” The HST data make the strongest case in strengthening the case for an exomoon. The authors believe that this shows the need to pursue similar Kepler planets for exomoons with HST and other facilities, even in cases where the Kepler data themselves do not show a large exomoon-like signature.
The paper is Teachey & Kipping, “Evidence for a large exomoon orbiting Kepler-1625b,” Science Advances 3 October 2016 (complete citation when I have it).
