What wavelength to best detect the “9th planet”?

What wavelength to best detect the “9th planet”?

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We know that the reflected sunlight will make detecting the 9th planet very difficult in the visible light. Is there another band that will be more likely to detect it? What is the surface temperature of this object likely to be, and what would that mean about its optimal detection wavelength?

Direct reflection of sunlight is the most likely scenario for a ninth planet discovery, however that does not hold if the object has a very low albedo. I assume you are interested in what wavelengths the planet would radiate.

For the surface temperature, the rotation of the planet is important. If it is locked with one side facing the sun, or rotates very slowly, the centre of the sun facing hemisphere radiates as much energy as it gets from the Sun. At 60 AU, the solar flux is about 0.38 W/m². Using the Stefan-Boltzmann law, we obtain a equilibrium surface temperature of 51 K (that is the highest possible surface temperature, assuming it does not have an atmosphere). Wien's displacement law tells us that radiation from a 51 k object peaks at a wavelength of 57 µm (infra-red).

For a rotating body, the equator temperature is 38 K, with radiation peaking at 78 µm (still infra-red).

Using an albedo of 0.5, the peaks are 68 µm and 90 µm for a non-rotating and a rotating body respectively. Note that this is for the equator region only, the actual peak-wavelength is going to be a little bit higher, belonging in the far infra-red spectrum. Also, the high uncertainty of rotation, albedo and mass (mass is important for internal heat), makes it impossible to get a higher accuracy than that

60 au is a very optimistic perihelion distance for the ninth planet, so for a more realistic distance of say 200 au, it is not possible to observe it in the IR spectrum, if it does not have a significant internal heat source.

The possible planet 9 is thought to be about 10 Earth masses and is unlikely to be a gas giant (it may be the core of an "interrupted" gas giant). As such, it will not be generating significant luminosity itself and would be rocky, or more likely, icy in character. It would thus only be seen by reflected light.

The considerations for what wavelength to search in balance the sensitivity of the instruments at hand with the likely spectrum of the object. This in turn depends on the solar spectrum and the wavelength dependence of the reflectivity (albedo).

For most icy objects, including Pluto and Trans-Neptunian objects, the reflectance increases to the red and near-infrared, whilst the solar spectrum peaks at shorter wavelengths. This suggests that searches are best carried out with wide field optical instruments in the R or r' bands at around 600 nm.

A further factor in finding a candidate is that you are going to have to cover a large area. This is only feasible at optical and NIR wavelengths unless the object was bright enough in the mid-IR to show up in WISE (which I'm sure is being thoroughly checked). A press release I saw said SUBARU is being used for the search. I would bet they are using the half degree field of Suprime-Cam at optical wavelengths and not pursuing COMICS mid-IR imaging with it 42x32 arcsecond field!

Confirming a candidate should be easy, given the enormous parallax and proper motion expected.

There are two basic ways to detect such an object. First is to detect it through reflected sunlight. Second is from the heat that it produces. We already know that the reflected light of such an object likely would be around a 16.5 magnitude. To determine the infrared, we have to estimate the temperature

The temperature very much depends on the composition. For simplicity, let's assume a composition similar to Earth, and was created about the same time as the rest of the Solar System. These assumptions may not prove to be valid, but they are among the possibilities discussed. Earth's internal heat, in fact, is at least 50% from radioactive decay, according to Scientific America. Of course, that's the internal heat only, not all of that will make it to the surface.

This proposed planet is somewhat akin to a "Rogue Planet", where a small disk of gas collapsed into a planet without a star, or were ejected from their host system. A fair bit also depends on if there is a sizable moon of the object. If so, then tidal heating would dramatically increase the temperature of the object. Any such determination can't be made without observation, but it is possible. An atmosphere would also help to keep the planet from freezing. A paper for detecting rogue planets comes from Abbott and Switzer. They hypothesis that a 3.5 Earth Mass object could be detected if it comes within 1000 AU, specifically in the far infrared, with a surface temperature of about 50 K.

Bottom line, it would probably be wise to try to detect both in the far infrared, as well as the visible, although it might be difficult to detect, even then. Given parallax as the primary means of motion, the detection should be done at several points in Earth's orbit, probably the same spot should be searched about 90 days apart to give the maximum opportunity to move, as parallax would only be visible if the motion of the Earth was perpendicular to the location of the object.

Ann I Zabludoff

Ann Zabludoff is Professor of Astronomy at the University of Arizona. A Pennsylvania native, she obtained bachelor’s degrees in Physics (1986) and in Mathematics (1987) from the Massachusetts Institute of Technology. She received master’s (1988) and doctoral (1993) degrees in Astronomy from Harvard University, where she was a NASA Graduate Student Research Fellow. She spent her postdoctoral years first as a Carnegie Fellow (1993–1996) at the Observatories of the Carnegie Institution in Pasadena, California, and then as a NASA Edwin P. Hubble Fellow (1996–1999) at the University of California in Santa Cruz. In 1999, she joined the Arizona faculty. She is a member of UA’s Data Science Institute, with interests in machine learning, image analysis, and large scale visualization.

Professor Zabludoff has led a wide range of studies across astronomy, astrophysics, and cosmology. Her work includes analyses of large observational databases and theoretical simulations, as well as the adaptation of astronomical instruments for new science. She and her collaborators discovered the role of groups in driving galaxy evolution (now called “preprocessing”), built a theoretical framework for multiple gravitational lensing planes, showed that distant Lyman-alpha emitting nebulae are tracers of cluster formation, clocked the time evolution of merger-induced galaxy transformations, quantified the connection between tidal disruption events and their host galaxies, accounted for nearly all of the baryons in cluster halos, proposed a new way of directly imaging exoplanets using eclipsing binaries, extended time-dependent, general relativistic “slim disk” accretion models to supermassive black holes for the first time, and upgraded the MMT’s SPOL CCD Imaging/Spectropolarimeter to map the polarization of the cosmic web. She leads the HotShots citizen science project to detect the sources of gravitational waves.

Professor Zabludoff was a J. S. Guggenheim Foundation Fellow in 2013-14 and the Caroline Herschel Distinguished Visitor at the Space Telescope Science Institute during 2011–2013. She has been an invited visitor at institutes around the world and has given review talks at more than 30 international conferences on a broad array of topics. She was plenary speaker at the 2019 Astronomische Gesellschaft and the 2018 Korean Astronomical Society meetings, as well as keynote speaker at the 2019 Advanced Imaging Conference. She discusses using compound gravitational lenses to detect the earliest galaxies in this TEDx talk. She has held leadership positions advising the NSF, NASA, and international research institutes on programs, facilities, hiring, and diversity, equity, and inclusion. She has mentored numerous junior scientists, including the 2019 ASP Robert J. Trumpler Awardee for best Ph.D. thesis in North America. She was the Graduate Program Director for UA’s Astronomy Department from 2005 to 2013, supervising 40-50 PhD students at a time. Her research is supported by the NSF and NASA.


  • Ph.D. Astronomy
    • Harvard University, Cambridge, Massachusetts, USA
    • The Kinematics of Dense Clusters of Galaxies
    • Harvard University
    • M.I.T., Cambridge, Massachusetts, USA
    • M.I.T., Cambridge, Massachusetts, USA

    Work Experience

    • University of Arizona, Tucson, Arizona (2011 - Ongoing)
    • University of Arizona, Tucson, Arizona (2005 - 2011)
    • University of Arizona, Tucson, Arizona (1999 - 2005)


    • Hubble Fellowship
      • NASA, Fall 1996
      • Fall 1995
      • Fall 1993
      • The Carnegie Institution of Washington, Spring 1993
      • Harvard University, Spring 1989
      • International Space Science Institute, Space Science Reviews, Spring 2020
      • Advanced Imaging Conference, San Jose Convention Center, Nov 15-17 2019, Fall 2019
      • Astronomische Gesellschaft, Fall 2019
      • Korean Astronomical Society, Spring 2018
      • John Simon Guggenheim Memorial Foundation, Fall 2013
      • TEDXTucson, Spring 2012
      • AURA, Fall 2011

      Related Links



      Teaching undergraduates and graduates, introductory and advanced courses. Mentoring undergraduates, particularly from disadvantaged backgrounds, as well as graduate students and post-doctoral researchers. Improving the graduate and undergraduate astronomy curriculum. Creating new undergraduate courses for non-majors.Improving the intellectual atmosphere via renovations of collaborative spaces and more faculty engagement.


      I have led a wide range of studies across extragalactic astronomy and cosmology, exploring the first generation of stars and galaxies, galaxy transformation via mergers, gravitational lensing, dark matter, supernovae, the intergalactic medium, galactic nuclear activity, galactic spectral classification, the baryon budget of the Universe, stellar disruption by supermassive black holes, and the evolution of large-scale structure. My collaborators and I discovered the role of groups in driving galaxy evolution in richer clusters (now called "preprocessing"), built a theoretical framework to calculate the effects of multiple gravitational lensing planes, proposed that Lyman-alpha emitting nebulae at high redshift were precursors of galaxy groups and tracers of cluster formation, uncovered the details of merger-induced transitions of galaxies from gas-rich, star forming disks to gas-poor, quiescent spheroids, quantified the connection between tidal disruption events and their host galaxies, found nearly all of the baryons lying in cluster halos, and, most recently, developed the first successful, fully general relativistic, slim disk model of tidal disruption event disks. I have worked to adapt a 4-Meter, Wide Field Coronagraph Space Telescope designed for exoplanet science for use in extragalactic research, to upgrade the SPOL CCD Imaging/Spectropolarimeter on the MMT telescope, and to develop the Multi-mode Integral Field Units instrument for the Magellan telescope (IFU-M).

      The big idea

      Students at Catholic colleges and universities begin their studies with more positive attitudes toward gay, lesbian and bisexual people than their peers at evangelical colleges and universities, our survey found. But that&rsquos no longer the case by the time they graduate.

      Multidisciplinary research teams at Ohio State University, North Carolina State University and Interfaith Youth Core, a Chicago-based nonprofit, surveyed 3,486 students attending 122 institutions of various types, sizes and affiliations. Our study, the Interfaith Diversity Experiences and Attitudes Longitudinal Survey, polled the students three times over their time in college &ndash in the fall of 2015, the spring of 2016 and the spring of 2019.

      We asked students whether they agree or disagree &ndash and how strongly &ndash with various statements about gay, lesbian and bisexual people. The statements related to, for example, whether students believe gay, lesbian and bisexual individuals are ethical people and make positive contributions to society. They also asked students if they believe they have things in common with this group, and positive attitudes toward them.

      To avoid conflating sexual orientation with gender, we asked separate questions about attitudes toward transgender people, who were not included in this analysis.

      We found that students at Christian schools &ndash whether Protestant, evangelical or Catholic &ndash entered college with less positive attitudes toward gay, lesbian and bisexual people compared with those at nonreligious schools. All students increased in their positive attitudes for this group by the time they graduated.

      However Catholic school students made the least gains. Upon entering college, their attitudes were more positive than evangelical students and showed an initial surge after the first year. Yet when they left college they had the least positive scores.

      In contrast, Catholic students across all the surveyed schools came to college with a higher appreciation for gay, lesbian and bisexual people compared to all other Christian students. And that appreciation continued to grow significantly over the four years, regardless of institution type.

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