Tue. Dec 7th, 2021

Title: A jovial analog orbiting a white dwarf star

Authors: JW Blackman, JP Beaulieu, DP Bennett, C. Danielski, C. Alard, AA Cole, A. Vandorou, C. Ranc, SK Terry, A. Bhattacharya, I. Bond, E. Bachelet, D. Veras, N. Koshimoto , V. Batista & JB Marquette

First author institution: School of Natural Sciences, University of Tasmania, Hobart, Australia

Status: Published in Nature [closed access]

Have you ever wondered about the fate of the Earth? Humans have only existed for a small fraction of the Earth’s lifetime since it formed. Although we can not predict what will happen on the earth’s surface, the planet as a whole is likely to survive billions of years into the future. The fate of the earth depends on the fate of the Sun, which brings us to the study of the evolution of the star. During its main sequence phase, the sun fuses hydrogen to helium in its core. When that hydrogen runs dry, the sun becomes one red giant. Its core will contract under gravity, and the outer layers will expand past Mercury. The Earth will most likely be engulfed by the Sun in about 8 billion years. Eventually the Sun will shed its outer layers and the remaining core will be a white dwarf, but the Earth would then be too far away. It’s a little sad to think that our planet will not survive the aging sun, but what about other planets further out in the solar system? What about Jupiter or planets beyond that?

Since we can not accelerate the evolution of the Sun, we can look for other planets around other stars that are in the late stages of their lives. If there are any exoplanets orbiting a red giant or a white dwarf, it will give us an insight into our own future.

How to find planets with microlensing

The three most popular methods of detecting exoplanets are radial velocity, transit, and direct imaging. With the first two methods, astronomers look for planet-induced periodic variations in the speed or brightness of the host star. The effect is more noticeable if the planets are larger, more massive, and if they orbit closer to the host star. The direct imaging method works best when the planet is large and orbits very far from the host star. These detection perturbations are not ideal when we want to find planetary systems like our own. For this reason, the authors turn to today’s paper for gravity microlensing.

That microlensing technique detects the magnification of a background star due to the gravity of an objective object passing in front of it (see this astrobit for an exoplanet study using this technique). When one star (lens) passes in front of another (source), the gravitational lens amplifies the light from the source. If the lens star has a planet in orbit around it and the planet is near the star’s Einstein ring, its gravity causes an additional increase in the measured intensity from the source.

Lens events are rare, but their occurrence is less dependent on the properties of the planet, giving us a more objective study of exoplanet populations. In addition, the lens technique is sensitive to Earth-like planets around cool stars. This was the first method that was able to detect planets with Earth-like mass around ordinary main sequence stars.

Figure 1: Schematic illustration of the microlensing technique. When one star (lens) passes in front of another (object), the gravitational lens increases the light measured from the object (part a). When the lens orbits a planet and the planet intersects the Einstein ring of the lens, its gravitational field gives another boost to the measured intensity. These extra boosts can be used to find new planets. Figure from Chambers (2010) and caption from Sukrit Ranjan.

The planet with a missed host

The authors of today’s paper discovered a planet using microlensing, but they did not detect light from a star host in the main sequence. The microlensing event in question, MOA-2010-BLG-477Lb, was found by the Microlensing Observations in Astrophysics collaboration in 2010. The researchers adapted models to the microlensing light curve assuming the host star is a main sequence star and found that the best fit is 0.15 to 0.93 solar masses. The most suitable solution also has a planet between 0.5 to 2.1 Jupiter masses. Considering the estimates of the correct motion of the lens star, the team was able to predict where it would move relative to the source star after the microlensing event. They used the Keck II telescope to obtain follow-up images, shown in Figure 2. The contours in panel c show the expected location of the possible main sequence host, but there is no star to be found! If there is no main sequence star but the mass of the star is known, then what can the host be?

Left panel: an image with three visible light points.  Center: a zoom-in view, with a star in the center and one at the top left.  Right: the same image with contours superimposed.

Fig. 2. Panel a: a 2015 image of the microlensing event MOA-2010-BLG-477. Panel b: a zoom-in view, the bright object in the middle is the background source star. The faint emission to the northeast (top left) is an unrelated star. Panel c: the same field in 2018, where the contours indicate the probable positions of a possible main sequence star host from the microlensing analysis, but no such host is detected in the image. Reproduced from Figure 1 in the paper.

The host can only be a white dwarf

The lensing analysis limits the predicted brightness of the lens star, which depends on the unknown lens distance. FIG. 3 shows that the range of possible main sequence lenses for the event would all be brighter than the Keck detection limit. Since no such star is observed, the lens cannot be a main sequence star. The lens star can also not be a brown dwarf because the lens system is at least 0.15 solar masses. Similarly, the upper mass limit of 0.78 solar masses excludes neutron stars and black holes as host stars. Since main sequence stars, brown dwarfs, neutron stars, and black holes are excluded, the authors conclude that the lens must be a white dwarf.

A plot with diagonal curves from bottom left to top right that intersect dotted lines that go from top left to bottom right.  There is a gray band at the bottom indicating the detection limit and the main curves are above it.

What does it mean?

Main sequence stars like our sun evolve wildly into white dwarfs. Our earth is unlikely to survive the red giant phase of the sun, however simulations predict that planets in Jupiter-like orbits can survive. This system is the first observed Jupiter analog orbiting a white dwarf, a proof that planets around white dwarfs can survive the gigantic stages of their host evolution. This system represents a possible final phase of the Sun and Jupiter in our own solar system.

Astrobite edited by Macy Huston

Featured Image Credit: JW Blackman

About Zili Shen

Hi! I am a Ph.D. student in astronomy at Yale University. My research focuses on ultra-diffuse galaxies and their globular cluster populations. Since I came to Yale, I have been working on two “dark-matter-free” galaxies NGC1052-DF2 and DF4. I have coped with the pandemic and worked from home by making sourdough bread and baking various cookies and cakes, reading books ranging from philosophy to virology, going on daily walks or jogs and watching too many TV shows.

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