Tag Archives: planets

HD 149382 b Does Not Exist

For his second-year project, graduate student Jackson Norris studied spectra taken with the High Resolution Spectrograph at Hobby-Eberly Telescope of the hot subdwarf star HD 149382.


Hot subdwarfs are a mysterious, rare class of very hot star (spectral type O or B).  A hot subdwarf is a helium burning star, meaning that it has already exhausted its supply of hydrogen and has left the Main Sequence.  On a cosmic timescale, it will soon run out of helium and become a white dwarf.
Normally, stars in this state of their lives are giant stars, but hot subdwarfs seem to have had their outer envelope stripped away, exposing only their hot, underlying material.  This makes them hotter, smaller, and much less massive than the usual helium-burning giant star.    They are called “subdwarfs” because they are less luminous than hydrogen burning stars of the same temperature (because those stars have significantly more mass, they are brighter and larger.  A “dwarf star” is just a hydrogen burning star.).
The mechanism for this stripping is unknown, since at least some hot subdwarfs appear to be single stars (normally, when a star is missing its outer envelope it is because a close binary companion has gravitationally stolen it away).  
There is some theoretical speculation that close-in giant planets could be the responsible parties for this stripping.  Planets, being much less massive than stars, could have escaped easy notice, but some studies suggested that they could still have enough mass (and orbital energy) to have stripped away the outer layers of a giant star, creating a hot subdwarf.
In 2009, Geier et al. published a paper entitled: “Discovery of a Close Substellar Companion to the Hot Subdwarf Star HD 149382–The Decisive Influence of Substellar Objects on Late Stellar Evolution”  They claimed to have detected a massive planet, 8 to 32 times the mass of Jupiter, orbiting the brightest hot subdwarf in the sky, HD 149382 (following convention, they named the planet HD 149382b).  This discovery, they claimed, proved the theory that close-in planets are the mechanism for the creation of hot subdwarf stars.
We were skeptical that the observations Geier et al. had made really supported their conclusion that the star was orbited by a planet.  So a team of us, including Prof. Suvrath Mahadevan and Prof. Richard Wade, applied for time on the Hobby Eberly Telescope to make our own observations of the star.  The spectra were reduced by PSU graduate student Sara Gettel and analyzed by Jackson Norris.  Mr. Norris determined that the star is not, in fact, orbited by such a planet, and that we can in fact rule out almost any short-period planet orbiting the star down to a mass of less than that of Jupiter. 
This means that the origin of HD 149382 is still quite a mystery.  
The preprint of Mr. Norris’s paper is here:


It has been accepted for publication in the Astrophysical Journal and should appear in the Nov. 20 issue.


Big news here at Penn State:  we have been selected by the NSF to build a new instrument capable of finding rocky planets orbiting stars at just the right distance to harbor liquid surface water!  Press release is here.

Penn State Awarded $3.3 Million to Build Instrument for Finding Planets in Habitable Zones Around Nearby Stars

University Park, PA — A new state-of-the-art instrument — a precision spectrograph for finding planets in habitable zones around cool, nearby stars — is being developed at Penn State with support from a new $3.3-million grant from the National Science Foundation. “This new Habitable Zone Planet Finder instrument will allow us to detect the existence of planets that are similar in mass to Earth and also are in orbits that allow liquid water to exist on their surfaces,” said Suvrath Mahadevan, assistant professor of astronomy and astrophysics at Penn State and a co-principal investigator of the project.

The Habitable Zone Planet Finder (HPF) is a large infrared spectrograph that will be used to analyze the components of starlight and to detect the slight motions of nearby stars caused by the gravity of orbiting planets. It will be approximately the size of an SUV, and will weigh over two tons. “We have designed the HPF to be able to efficiently observe small, cool stars, which are by far the most numerous types of stars in the Milky Way galaxy,” explained co-principal investigator Lawrence Ramsey, professor of astronomy and astrophysics.

“These stars, which have temperatures far below that of the Sun, radiate very little of their energy in the visible part of the spectrum, so we must create an instrument that can capture the infrared part of the spectrum — where the unaided human eye cannot see but where these stars are brightest,” Ramsey said. The team chose to observe these stars because they offer the best opportunity for finding planets with solid surfaces in the so-called “Habitable Zone” around nearby stars, the range of distances from a star within which temperatures might be right for liquid water.

Hibby-Eberly Telescope

The instrument will take three years to build. When it is completed, it will be shipped to the Hobby-Eberly Telescope at McDonald Observatory in west Texas to begin its multi-year quest for new worlds, during which it will survey more than a hundred nearby stars. “This instrument will precisely measure the motion of each star, hunting for the telltale “wobble” caused by an orbiting planet,” said Penn State Assistant Professor Jason Wright, a co-principal investigator on the HPF team. “Once we detect these wobbles, we will be able to infer the surface temperature and mass of the planet or planets orbiting it. These cool stars we are targeting are some of the very closest to Earth. In astronomical terms, they are in our back yard.”

Associate Professor Steinn Sigurdsson, who is also a lead investigator of the Penn State Astrobiology Research Center, said “The HPF is a significant step toward discovering terrestrial planets in orbits where liquid water may be present on their surface around our closest stellar neighbors. This goal is important for the NASA astrobiology program since life, as we know it, needs liquid water. Nearby planets with liquid water on their surface are the first place to start looking for signatures of life on other planets.”

Because the planets the astronomers are looking for are so small, and the stars they are looking at are so cool, they need to build a new kind of planet-finding instrument. “To find an Earth-like planet around one of our target stars requires that we be able to measure the velocity of a star, located at a distance of tens of light years from the Earth, to a precision of about three feet per second — the typical speed a person walks — over a period of several years. To attain this goal, HPF not only must have high-efficiency optics and detectors, but must be maintained in an extremely stable environment,” Ramsey explained.

Mahadevan said the HPF team is keen to embark on this challenge. “I do believe we now have the technological capability to make these extremely precise measurements in the infrared,” he said. “Finding Earth analogs around our nearest neighbors will be an exciting adventure. We are grateful to have had prior support since 2005 from the National Science Foundation, the NASA Origins program, and the NASA Astrobiology Institute for the technology development that lead to our now being able to develop this instrument.”

The HPF development-and science team also includes Penn State Evan Pugh Professor of Astronomy and Astrophysics Alex Wolszczan, discoverer, in 1992, of the first planets ever found outside of our solar system, and director of Penn State’s Center for Exoplanets and Habitable Worlds; Distinguished Professor of Geosciences James Kasting, who performed some of the first calculations on Habitable Zones around stars; Mike Endl and William Cochran, researchers at the University of Texas; Fred Hearty of the University of Virginia; Penn State Postdoctoral Fellows Chad Bender and Rohit Deshpande; as well as former Penn State graduate student Stephen Redman and current graduate student Ryan Terrien.

The William P. Hobby-Robert E. Eberly Telescope – one of the largest and most powerful telescopes in the world -was conceived in the mid-1990s by Lawrence Ramsey, former head of the Department of Astronomy and Astrophysics at Penn State, and Daniel W. Weedman, then a professor of astronomy and astrophysics at Penn State. The Hobby-Eberly Telescope (HET) is located at the McDonald Observatory in a remote area of West Texas, where night skies are among the darkest in the continental United States. The Hobby-Eberly telescope was built by a partnership of five universities: the University of Texas at Austin; Penn State University; Stanford University; and two German universities, Georg-August University in Goettingen and Ludwig-Maximilians University in Munich.